First, both BG1 and BG2 are amazing games. I personally like BG1 over BG2, which I finished 5 times.
About your questions, short answer is, interface is very simple, combat is straightforward (except magic but will come to that), and you almost always know where to go.
Now that this is out of the way, I want to mention what makes BG1 not so newbie friendly. I will mention my own experiences.
First, for me, BG games (based on D&D 2nd edition) have the worst ability, chance to hit, armor system in the planet. After you learn that, you learn half of the game. Generally, LOW
values mean BETTER
AC (Armor Class): Low means better. You have a basic AC of 10. DEX, armor and spells help decrease that value. When you are comparing two armors, lower means better.
THAC0 (To Hit Armor Class ZERO): Again, lower is better. You start the game with 20 THAC0. Which means you need to roll 20 (aka critical) to hit an opponent with 0 AC. STR helps melee THAC0 and DEX helps missle THAC0. Also, your THAC0 decreases as you level up based on your class. http://baldursgate.wikia.com/wiki/THAC0 NOTE that 20 always hits and 1 always misses.
The general rule is Minimum roll required to hit = (THAC0 of attacker)-(AC of target +Armor Class Modifier)
Which means, if you have 18 THAC0 and opponent has 10 AC, you need to roll minimum 8 (18-10=8) to hit them. Rolls are D20 which means a 20 side dice (aka 1-20)
Just keep in mind that, LOWER MEANS BETTER.
About weapons: D6 means a 6 side dice, aka 1-6. D8 means a 8 side dice aka 1-8. D6+1 means 1 added to the roll, which means 2-7. 2D6 means you roll 2 6-sided dices, which will mean 2-12.
Now you know what THAC0 and AC means. The other complex thing in BG games are the spells. D&D has a very unique way of using spells. This does not matter much in BG1 where you will be low level (below 7) but extremely important in BG2 (above 7). If you want a Wizard, take your time reading spell descriptions.
----------------- Quests and what to do next.
I consider BG1 an open world game where you can go anywhere you want. Usually, you will encounter people, mainly .. an old man... who will give you some ideas where to go. You will find letters which will give you more clues. Chapters also will give you ideas.
For me, the most important rule of Baldur's Gate is: SAVE OFTEN. Q key is your best friend in that game which will do a quick save. ALSO, I suggest you make SEPARATE game saves as you progress in the game, mainly when enter a new area.
Exploration is the fun part of BG games. Explore the whole area. But remember. ANYTHING can kill you in this game. Especially in early levels where your health is pathetic. Yes, there are easy enemies and harder enemies but since 20 always hit, a group of 2-3 goblins or xvarts can kill a party member with a good (for you bad) luck. In my experience, especially in early stages of the game, archers are the safe way of playing this game.
----------------------------- CHARACTER CREATION. Abilities
and Proficiencies are the most important part of a character. If you don't know what you are doing, you can easily mess it up. Usually, a roll of 80 is enough for most classes. Note that, higher abilities (stats) dont always mean better. If you look at CON (constitution) there is no difference between 7 CON and 14 CON. Or 7 DEX (dexterity) or 14 DEX if you are not a thief. Read the link and read the manual for more information.
Proficiencies (Skills) mean a lot in this game. For every character, I suggest having a "ranged" proficiency. I found that Bows are the best in this game since they shoot twice in a round. If you have high STR, slings are the second best since they benefit from the STR damage bonus unlike any other ranged weapons.
Mages can only have a sling anyways so if you are going for a mage, I suggest you take a sling Proficiency. It may sound cool to wield a staff or a dagger but with low health and high AC (remember, high is worse) enemies will tear you apart. So, a mage should be smart and sling from behind. Of course, mages can be turned into a killing machine or a tank in higher levels but that will be in BG2.
I believe that is all you need to know to have the most fun out of BG. Do that. Have fun... your way.
EDIT: Changed professions into Proficiency. Thanks Bookwyrm627 for the correction.