There are several modding guides for BG1 out in the wild, but most expect a Tutu or BGT installation.
While both are very worthwhile projects with some wonderful mods specific to them, they aren't flawless conversions and they introduce a lot of additional bugs and gameplay imbalances. For better or worse (and *many* feel it's better), the BG1 experience just isn't the same.
I'm writing this modding guide to provide a much more conservative take on modding BG1--to provide an enhanced game that is suitable for first-timers and people who want to keep the "feel" of BG1 intact.
Highly Recommended Mods:
Widescreen Mod and GUI Mod
Be sure to use one of the following resolutions when installing widescreen or the installing the GUI mod will fail: 1024x600, 1024x768, 1280x768, 1280x800.
To install the GUI mod, unpack the RAR archive you downloaded, run the executable and point it to your BG1 directory, the go to that directory yourself and run Setup-TWM_GUI.exe
Baldurdash bugfixes and text update
Bugfixes and spelling/grammatical errors fixed. Adds some textual hints about hidden items as well, which is nice. Install the TOSTC versions.
Adds additional quests, features and areas. Very high quality, and blends with the original game perfectly.
Mini Quests and Encounters
Adds a lot of very small but very fun and generally well-written quests.
Herbs and Potions add-in
A little extra fun for crafters. A reasonably balanced, very low-key mod most people won't even notice. Recommended, but only if it appeals to you.
BG1 Tweak Pack
Tweaks, fixes and cheats. I recommend the following:
Weapon Animation Tweaks (less repetitive sword swinging)
Icewind Dale Casting Graphics (uses IWD spell effect which are much prettier!)
Icon Improvements (replaces some icons with their usually more detailed BG2 equivilients)
Force All Dialog to Pause (No cheap shots from enemies while you are talking)
Reveal City Maps (Probably don't use this your first game if you like the feel of exploration. Removes a minor annoyance if you have played before)
Use your discretion.
Convenience Tweaks and/or Cheats:
Does what is says on the tin. Higher HP on level up may be worthwhile for some to avoid some potentially bad rolls on BG1's random HP gains on level up.
Joinable NPC Tweaks:
Give <character> their BG2 stats isn't the biggest cheat ever and can be fun for some people. Everything here is highly subjective of course.
Big, game-changing mods or highly subjective tweaks that are worth checking out.
Fields of the Dead
Reworks the ruleset quite substantially to be much closer to AD&D 2E. Adds new quests, areas, spells and more. Quite high quality. I don't use it for every playthough, but it was interesting and well worth at least one.
Dark Side of the Sword Coast
Massive mod, adding tons of new quests, areas, spells and lots more. One of the first mods for Baldur's Gate. It used to be a bit of a mess, but it's much, much better than it used to be thanks to recent efforts to fix bugs and increase compatibility with other mods though use of the (now universal) WeiDu installer.
The quality is of quests and writing is quite variable, some items are cheese-tastic and there are still a few bugs lurking, but under all that, it's quite fun.
If you're craving more content, this mod has you covered.
Adds a lot of challenging encounters to the game and a quest or two. Not for everyone, but veterans who enjoy tactical combat should give it a go. I don't think I've experienced the full content of this mod, but I liked what I saw.
The Stone of Askavar
Adds a massive new quest. This one isn't a recommendation as I encountered a lot of bugs and even a couple crash-to-desktops using this mod. I also remember there being a lot of mispellings and grammatical errors in the text. So why mention it? There's a lot of content here and not everyone will share my misgivings. If you're craving more Baldur's Gate, back up your installation and see what you think.
BG1 NPC Portrait Pack
If you don't like the original portraits, or just want to freshen the look of your game a little, check out this collection of high-quality alternative portraits for the games' NPCs.
May be of interest to some, but I can't vouch for them.
A more comprehensive fixpack than Baldurdash. Not as popular so fewer mods are coded to recognize it. I've read that compatibility should be decent, but don't know firsthand.
Back to Baldur's Gate
A "harder enemies" mod that may appeal to tacticians.
The Drizzt Saga adds a quest involving everyone's (least?) favorite Drow.
The Grey Clan Episode I
The first part of a mod trilogy that's isn't likely to ever be completed. This mod has a decent reputation however, and may well be worthwhile.
Pocketplane keeps an updated index of many known Baldur's Gate mods.
rartino wrote a guide with a similar purpose that explains some things in greater detail and has some great troubleshooting tips, you can read it here.