As someone who has beaten the game multiple times, here's what I recommend:
* Be careful if you use the "Modify" button. Making your party too powerful will make the encounters much more difficult. Fight the urge to max out your stats.
* Read as many of the rules as you can in the rule book. Understanding AD&D helps, and the books do a good job of teaching you what you need to know.
* Learn what all the spells you pick can do, as casting the right spells at the right time can save you when things are going bad.
* The only way to make the game unwinnable is if you only have one save and lock yourself into a place you can't escape. You can easily win this game with a non-optimal party, so play whatever you want. Just save often and use multiple save slots, and you should be fine.
* This game doesn't require Gold-Box Companion (none of the Gold-Box games do). People will tell you it is "essential," but people are dumbfucks. These games aren't that hard once you grasp the system. Consider playing without it for a pure experience. We got along fine without it for years.
* Dying in combat will happen. Don't let it frustrate you. Expect to see the monsters rejoicing at your defeat on occasion. Reload and try again.
* Enjoy the experience. These days, you can beat the game in less than a week. Playing on a Commodore 64 back in 1988 with no FastLoad cartridge and only a couple of hours a week, it took me 9 months to beat it...but I still loved every moment of it.