Good longboarding forum would be silverfishlongboarding.com.
Boards: Different shapes are all about preference honestly, just little changes in things like concave (the shape of where your feet go on the board), flex, and that kind of thing. Drop through vs top mount decks have always been a pretty good back and forth argument. Drop through decks will be able to initiate a slide easier, however top mount decks will give a more controlled feeling while sliding. Drop through decks are also closer to the ground which can make pushing a little easier. Honestly both work and you will just have to ride boards to learn what you like. It should be noted that with just about all drop through decks, you can top mount them also.
Trucks: The degree of the trucks is the first thing to note. the higher degree the truck, the more the truck will turn the board when you tilt/carve the board. Less degree angle, less turning, its that simple. Bushings in trucks are extremely important. I haven't messed with bushings alot yet but I do know what different bushings do. There are two main types of bushings, cones and barrels. Cone bushings are cone shaped and will dip and dive into a turn much easier than a barrel shaped bushing. This makes them good carving bushings, however they will not have a ton of rebound back to center. Barrel bushings will be harder to carve (note its not that big of a deal) and they will rebound significantly more than a cone shaped bushing of the same hardness bent to the same position. Barrel shaped bushings will provide a stiffer feeling board, which can result to a feeling of more stability.
Wheels: This could turn into a very long paragraph but ill keep it short. Soft wheels ride smoother but will roll at a slightly slower speed. Hard wheels will roll faster and be a little bumpier and generally slide easier. Contact patch is the width of the wheel that contacts the ground. The wider the contact patch, and the softer the wheel, generally the more grip the wheel will have. However each kind of wheel and urethane formula will perform and react differently.
If you have any examples of the kind of riding you wish to do, just post a reply and I will try my best to explain anything you want.