This is the sand-paper-like layer, usually black, that is on the top of the deck (find out more). One sheet is all you need to cover your board. There are slightly better, finer grip tapes available, if you want. It all depends on how much you want to spend on your board. At skate shops or online, you can often have them put the grip tape on for you, but you can also apply the grip tape yourself, and make your own personal designs. It's fairly easy - read How To Apply Grip Tape to a Skateboard Deck.
Risers do two things. They help relieve stress from the trucks, which helps keep the deck from cracking. More importantly, risers help keep the wheels from biting into the board on a hard turn, causing the board to suddenly stop. It's a bad thing to have happen. Most risers are about 1/8" high. If you have extra large wheels, you'll want higher risers. On the other hand, if your wheels are small (52mm), then you might not need risers at all. It all depends on what you want.
The nuts and screws to put the board together. There are special colored nuts and bolts available, if you want. This is all just for looks -- if you're on a budget, just get the basic parts.