Star Wars: Knights of the Old Republic is a huge game. Especially when starting out, you will get lost. Here's some tips to help you out on your way early on.
Choose wisely, padawan.
Choosing a class and starting attributes without having ever gotten to play the game is rough, because it really takes getting in to the game and playing it a lot to realize just what is possible, and the path you might want to take with your characters.
If you're the type that prefers beating people down and being the tank, then the Soldier class is for you. Scoundrels are best at most non-combat abilities like Persuasion and Security, meaning Jedi mind tricks and breaking in to doors. Scouts provide a balance between the two. However, there's two things to consider when choosing a class early on: one, it's possible to travel with a party of three, so even if one play style is preferred, it's possible to have party members fill a different but useful role. Two, the main character is always gonna be there, and there's certain skills like Persuasion that only they can really use. So, why not have them focus on that? Use the wookiee to beat stuff up.
Focus, you must.
Jack of all trades, master of none. That saying is a warning in KotOR. You're going to get the best effects and stat bonuses by choosing to focus on particular paths, whether it be a certain set of combat skills, or going down the Dark or Light Side path. Have a plan and stick to it and you'll get the best results. Again, if you're worried about any deficiencies, remember that you have three party members and can control any individual character, so you can cover most any area of expertise at any time if your party is set up accordingly.
Blow it up like Alderaan.
Your first playthrough is going to be one giant mass of confusion. Taris, the first planet, is big and confusing and you will get lost often. Plus, you won't quite know what everything in and out of combat is until you experience it in the game.
By the time you're ready to leave Taris, you've gotten to experience the gist of the game's elements, so this is time to make a decision: do you want to keep going with your selected main character? Don't be afraid to start anew at this point. It will take significantly less time to get back to where you started, and odds are, you'll be far more satisfied with the results. And if not, hey, there's multiple save slots for a reason. Use them!
Exploit the game like the Death Star's exhaust port!
Need a quick heal? If you can warp out the area on the map screen, you can get healed instantly and for free. And unlike on the Xbox, loading is so brief that you'll save yourself thousands of credits on medpacks.
If you have a tough Persuasion roll, know that some characters can be re-rolled by going through the conversation again. This doesn't always work, but consider it. Just quicksave before making any important decisions.
The structure of the game isn't perfect - you'll discover the gaps and how to take advantage of them as time goes on.
Google is more powerful than the Force
This is a console/PC game from 2003 that's based on Dungeons and Dragons which is even older than that. There is a lot of info out there. Don't be afraid to use it if you're confused or get stuck. StrategyWiki's KOTOR guide has a fantastic database of numbers and information. IGN has a great guide with plenty of visual aids if you get stuck. GameFAQs can pretty much fill in the rest, because hey - it's GameFAQs and KotOR was originally released in the prime era of GameFAQs' existence.
Have fun.
Of course, part of the fun of KotOR is the ability to dig in and find a lot of this stuff out yourself. Don't just follow the guides. Experiment! The quicksave and multiple save slots are there for a reason. Explore! There's a lot to find, and when you discover it yourself, it's the most satisfying.
