This is merely a write-up/walk-through for how I generally approach Baldur's Gate 1. It's primarily for my own recollection, but if others wish to use and review it, they're certainly more than welcome to. To best describe my play-style, I always like to play core rules, but admittedly hate the few negative side effects that come with it (no auto-writing scrolls, no auto-max-health levels, etc.) ...so, while I certainly won't be "cheating" by using console commands or anything, I'll likely take the time to plan and act out the most advantageous approach.
I practically always go with a Human Warrior (Fighter or Ranger), generally without any specialty kits as I can never get over how extreme the disadvantages seem, but I do like to consider eventually dual-classing to a Cleric, probably around lvl 9 as I'll get the most out of the warrior health bonus. If not, I only really have to worry about three of the six stats (Strength, Dexterity, and Constitution), and I generally go with the proficiencies Longbow and Long Sword (Composite Longbow +1 / Varscona). If so, I have to also worry about a fourth stat (Wisdom), and with a Cleric's weapon set in mind I generally go with the proficiencies Sling and Warhammer or Flail/Morning Star (Sling +1 / Ashideena).
There are two stats that are important to practically every class; Dexterity, which helps missile-weapon THAC0 and Armor Class, and Constitution, which helps overall survivability, especially for warrior classes and "shorty" races. Otherwise, a high Strength improves weapon damage and melee-weapon THAC0, Intelligence benefits the mage-spell user, Wisdom benefits the priest-spell user, and Charisma is useful for the party leader during certain interactions with the friendly populace - pointedly useless for your PC if they won't be the party leader. If it's an important stat, I'll push it to 18 in order to have every advantage, and if it's not important I shoot for 10 to avoid any disadvantage, even though most significant disadvantages start below 8. However, if dual-classing to a cleric and following my guide, one would need to roll at least a 92 on stats (18/**-18-18-10-18-10) ...though I've personally rolled a 96 before (as a Paladin, 94 as a Ranger, and 92 as a Fighter), so while this shouldn't be impossible, it will likely take a lot of time and patience.
Most veterans to this game can run through this blindfolded in under 30 minutes, but I'll leave some advice here anyways. First, loot everything you can without getting caught, as gold is particularly valuable early on. Second, despite Gorion's advice, buy as little as possible; a missile weapon of your choice + ammunition, a large shield to accompany a 1-hand melee weapon if capable ...and that's probably it - you'll soon discover almost all the other weapons and armor you'll need. Third, complete all the quests; buy 20 bolts to save yourself some time and work your way counter-clockwise around the castle. By the end, you should have nearly 1000 exp, a few hundred gold, and a familiarity with everything related to Candlekeep.
Immediately after the nights events, you're approached by your first companion, Imoen, who will join your party after some brief dialogue. Whether or not you want her as a companion, it's imperative you delay that conversation and out-run her; head north to loot the items there, head east to pick a diamond out of a tree, exit your current area to the east, and find a magical ring within a nearby rock in the next area. From here, no matter what you plan on doing and who you plan on recruiting, your first goal should probably be to reach the furthest experience benchmark that a newly recruited companion will auto-level to. One may certainly recruit all the companions they'd want immediately without worrying about this, but you'll just end up having to earn up to 6x more experience as that party than you'd otherwise earn by using just your PC. As your party gains experience, new companions will make an attempt at matching your current average experience. As of writing this, the EE currently has several benchmarks; 2000, 4000, 8000, 16000, and 32000. So, as the highest benchmark is 32,000 experience, you should work towards reaching this as fast as possible, which brings us to...
Otherwise known as the area east of the Beregost temple, it contains the highest concentration of earnable experience available immediately to the player; a simple reputation-boosting quest (requiring a Stone to Flesh scroll), some agitating NPCs with noteworthy equipment, and several very dangerous creatures known as Basilisks. The Basilisks' strength comes from their ability to change most creatures to stone at a glance, something that a nearby friendly ghoul Korax is immune to, but he won't stay friendly forever so using him efficiently is essential. All told, this area alone should yield ~30,000 experience, pointedly bringing you very close to (if not past) the aforementioned benchmark.
After reaching at least 32,000 exp, one may recruit whomever they wish and continue with the game, but per the canon and given her abilities, I usually go with Imoen. She will be a level 6 Thief with only 8000 experience until level 7. This introduces the next challenge; she is best dual-classed to a mage at level 7, which locks out her thieving skills for the next 90,000 experience that's attributed to her. So, assuming there is some thieving that needs doing (looting towns, pick-pocketing important people, clearing trapped dungeons, etc.), it's generally good to try and get it all taken care of before the party earns too much more experience overall. However, after she hits level 7 and is presumably immediately dual-classed, it again becomes beneficial to get her leveled up as soon as possible, so it's best not to recruit anyone else (as you're already splitting experience two ways - between her and the PC), and hit every pre-populated high-experience endeavor that's available to you. This includes completing any simple quests you may have been holding out on, traveling out west (Lighthouse, Shipwreck's Coast), traveling around the south-east (Gullykin, Firewine Bridge, Valley of the Tombs, etc.), not to mention (if you have the expansion installed) hitting the first few levels of Durlag's Tower - just the exterior should yield ~30,000 exp. These encounters will most likely be quite challenging, especially with your PC doing most of the work, but potions and magical equipment will go a long way. In the end, you may not be able to reach Imoen's new goal before Chapter 4, but the closer you get, the sooner you'll be able to use her thief skills again.
Lastly, before clearing out the Nashkel Mines (starting Chapter 3), one would need to recruit Jaheira and Khalid from the Friendly Arm Inn if there's a desire to use them as companions, as they'll disappear from the game otherwise. In order to prevent this, one may simply recruit them at any point after reaching 32000 experience and immediately dismiss them for Imoen's sake. You may also use them for item storage, as you'll likely find several significant items that you won't immediately need but shouldn't sell either.
This part of the game basically just includes the Bandit Camp and all of Cloakwood. The Camp has several locks and a couple traps, so having a Thief would be beneficial, but a pre-dual-classed Imoen is unlikely to make it this far. Cloakwood is extremely difficult without a full party of six, and the mines at the end could really use a skilled Thief, but Imoen may still not be ready by this point - though perhaps you've come across a random-treasure-drop featuring the mage scroll Knock, despite it being readily available in the city of Baldur's Gate. Otherwise, I have no further advice, and you may certainly proceed as you wish...