Your best bet would to be to find a skate shop that can help you out. It's been years since I've skated but I'll help where I can.

How do I pick out a skateboard: mostly just aesthetic preference, most of the boards are basically the same size so the main thing that varies is design.

What do I need: Well if you're going all out I'd get some skate shoes as they grip the board better. If you're just starting out I'd recommend a helmet and wristguards at the very least, elbow and knee pads if you want to be really safe.

Do the wheels come with: There are fully assembled boards with wheels and trucks already attached, or you can pick them out individually. The skate shop will assemble it for you if you decide to buy a deck/trucks/wheels separately. (trucks are what attach the wheels to the board)

What's the difference between a regular skateboard and longboard: Longboards are made more for speed and travel rather than tricks/ramps. Regular boards take more energy to move on but their smaller size makes them easier to maneuver and perform tricks with.

What's the upkeep: Not much, skateboards are made to take a beating. If you're using it constantly you may need to replace the wheels every few months but if you're using it sparingly you shouldn't really have to worry about upkeep.

I haven't skated in years so there may be some new things I'm not aware of but I can't imagine all that much has changed.