The stats (AKA: Strength, Wisdom, etc.) your main PC has can have a strong effect on how deadly s/he is. A warrior with a 14 Strength for example, has no bonus modifiers for melee combat, while an 18/00 Str gives a whopping +3 to hit and +6 damage. Clerics with high Wisdom get a few bonus spell slots. Thieves with high Dex can use their thief skills even better. If you don't know anything about D&D, RTFM and learn what the stats do, and which stats have even greater importance for your class(es). Don't overlook the tables at the end of the manual.
I'm an old school gamer, so I play with total stats of 63 or so, but most gamers play with 80+, 90+, or even maxed stats (cheating). Depending on your playstyle and your views, you might want to spend a few minutes rerolling starting scores to get a high total for your main guy. You should also know that while your stats may alter a little through the game, for the most part, what you roll to start with is what you're going to have the entire series, so make sure you know what stats you want, and which are best for you.
2) Missile > Melee
It's a general rule for most games (hell, life!) that ranged attacks are preferable to close combat. If you get up close and personal, your enemies can hurt you and make you cry. Attack from afar, and they can't even bleed on you. In BG, bows give you two attacks a round versus one attack for anything else. SInce more attacks is a good thing, make sure all your guys have ranged weapons (don't forget classes have weapon restrictions!), and get bows for those who can use them. Make sure you have plenty of ammo too. Ammo is dirt cheap and weighs nothing, so stock up. RUN from anything that can shoot back.
3) Stay Standing
In BG, there are two things you need to stay up during combat. You need a good Armor Class, and you need good hit points. Good saves are also useful, but they come into play later. At the start of BG, it's all about AC and hp. Because warriors can wear the best armor and use the largest hit die for health, it's for this reason that Fighter is the most commonly recommended class for newbies. I'm not saying you need to play a Fighter, or a warrior at all, if that's not your style (I'm a caster myself). However, if you're tired of dying over and over and over and over and just want to play the damn game already before you chuck your computer out the window, then Fighter (or Paladin, Ranger, or Barbarian) might be for you.
Whatever class you choose, get the best AC you can, and hope for good rolls for hit points. Because PC death is game over, keep your guy away from the front line unless s/he's needed. One simple trick is to send other warriors in first so that monsters engage with them first, and THEN send your guy in to flank, or rear attack, or whatever. This will reduce you being targetted overly much. Remember, it doesn't matter if all your other NPCs die, so long as you live. There are puh-lenty of other NPCs to recruit in BG1.
While in the game, go to Options ("o") and then Gameplay. From there, click auto-pause. This window gives you all the options for when the game will instantly pause when a certain event occurs. Clicking on any of the words will give you a description at the bottom of how each option works.
Play around with them all and see which work for you during the game. While you might want to click all of them, you may find that the game constantly freezing comes to be rather irritating. Pretty damn fast. For myself, I only have "center on member", "enemy sighted" and "trap found" selected. Figure out which you like.
The most important one is "enemy sighted". Rather than blundering into a pack of monsters cause you were looking at your phone, the game will handily freeze for you when you come across an enemy. This can give you a chance for your party to back away before you're seen, but this isn't always the case. HOWEVER, if your scout has stumbled across enemies, then she can easily retreat (or scout further) without risk of being immediately sighted. Scouting and auto-pause together will go a long way towards helping you avoid fights you don't want, and that's something that is VERY important for you while you're low level.
5) The Start
Candlekeep is there to make sure you know the basics of combat. Don't forget to try out the training party if you want more practice. Do you know where to look to see what your scores for combat are like AC and hit bonuses? Are they as good as you can make them? Try swapping out armor and weapons to see how the numbers change.
When you've left Candlekeep and Imoen's joined you, don't wander! You are a tender little rosebud, and anything can squash you. Head due east ("dinnae strayyyy frim the path, laddie!"), and then follow the path north. This is a perfect time to practice scouting. Thieves have the ability to Hide in Shadows (HS). Imoen's a thief, have her try. Don't worry that she won't succeed initially. She's only 1st level, and sunlight gives a HS penalty, and depending on what mods you have, her armor might give a HS penalty also. When she fails, you'll have to wait six seconds (one round), to try again. Keep trying until she's stealthed.
Have her poke around a little, scouting the path east of where she joins you. Keep an eye on the text and notice that the game keeps checking to see if she stays stealthed. Because her scores are low, she won't stay stealthed longl. After reappearing, Hide again. Get a sense of how long it takes before she can Hide, how long it usually lasts, and once her stealth ends, how long until she's solid again. Knowing this will give you a good sense of how far to explore areas, how close you can get to monsters, and how quickly you need to run away.
Have Imoen scout the path east and then north. Once you've established an area is safe, have your PC move up. Be aware that once Imoen spots monsters, even though they can't see her, they might start wandering around, so don't let your visible guys get too close to monster spawn points. Remember, you're a delicate spongecake... give monsters a WIDE berth for now.
Follow the path up with Imoen scouting until you find Xzar and Montaron. Have them join. Now you are four! Your chance of survival just doubled! Go east and change maps. Note where the FAI is.
After dealing with the pompous old kook, have Imoen or Monty scout. Look at their scores, who is better? What if you took off their armor? Do you want to risk scouting without armor? Scout the area until you can find a safe path to get you to the Friendly Arm Inn. There are tons of brutes here and your party can easily die, so be very careful. Remember, delicate truffle. Combat is NOT your goal right now, survival is. Avoid everything, and run away if you can at all.
Once at the FAI, there's only one last hurdle, but it's a doozy. Quite a few gamers consider this fight one of, if not THE, hardest in the game. It's not that your enemy is powerful, it's that you're so weak in comparison. MANY PC deaths have happened here. FYI, some mods (like SCS) change the spells and tactics here, which may in fact make it easier. No more 6-15 dmg Magic Missile attacks is a great thing.
Remember what we've learned so far: scout/ ranged weapons/ warriors up front. Imoen has a wand of magic missiles ("I dunno how it got there. It must have fallen into my pocket!" (shut up Tas!)).
Her wand can help interrupt spellcasting, as can missile fire from the group. All that really matters is that the PC survives. If you can't accept any party deaths, you can reload. Or, you can accept their deaths and move on. Again, depending on mods, the local guards might or might not help, but don't count on them. They come with Stormtrooper skills, so if you actually see them hit once, buy a lottery ticket since you're obviously swimming in luck.
The wand will help you interrupt spells. Missiles will help. Melee can help too. Further, you might want to give Xzar the wand and have Imoen and/ or Monty try a backstab. If you don't know (didn't you RTFM yet?!), backstabs cause extra damage but can only be done by thieves while stealthed. Maybe after scouting the area, having a stealthed thief around to Stab here might be helpful. It all depends on your tactics and style.
Once you're inside, pick up Khalid and Jaheira. Now you are six! Make sure J is always stocked with healing spells, make sure they have ranged attacks, and then scout your way down to Beregost then Nashkel. Explore the maps on your way down (you can explore the first map where you got Imoen now if you want), but keep heading south. Rest a lot. Be aware though that if you rest too much, your teammates will start grumbling about your delay. They all want to go to Nashkel. If you take too long (two weeks? don't remember) they'll leave. So rest a lot, but don't make a vacation of it. You don't actually have to do anything in Nashkel. Just entering the map should be enough to end their complaints. Work on killing things and getting xp to level up. Note which monsters "pay" more for xp, and try to kill a lot of those. ALWAYS be prepared to run away ("Brave SIr Robin...") and have your escape route planned.
You should be L2 or even L3 before entering the Nashkel mines. If you have mods like SCS, higher still. Beware of kobold archers, particularly Commandos!!! Be 'ware! Make sure your casters have area spells to disable enemy shooters.
Good luck !