I am not familiar with any of the Pathfinder Adventure Paths, but Paizo is known for their great adventures, so I am sure Kingmaker will be good. Pathfinder is close enough to D&D 3.5 that there shouldn't be a big learning curve for you.

As a DM, I've only done a few sessions in 3.5, and I'm currently running a 4E game, but the basic principles are basically the same. The Golden Rule (have fun) was mentioned above, and that is the most important part, of course. Rule Zero is important, too, which states that the rules are just guidelines and you shouldn't let the rules get in the way of your and your players' having fun. Learn to say "Yes" to your players when they want to try something that isn't necessarily covered by the rules, but don't be afraid to say "No" when what they want to do will derail the game or diminish the fun of others.