There are three things to pay attention to:
You want to match the width of your trucks to the width of your deck. Match your truck side to your deck with the following chart:
4.75 for up to 7.5" wide decksYou'll want your trucks to be within 1/4" of the size of the deck.
5.0 for up to 7.75" wide decks
5.25 for up to 8.125" wide decks
For 8.25" and up, you can use 5.25 trucks, or use super wide trucks (like the Independent 169mm)
Inside the trucks are the bushings, a small part that looks like a rubber donut. The Bushings cushion the truck when it turns. The stiffer the bushings, the more stable the skateboard. The softer the bushings, the easier the turn. For a brand new skateboarder, I recommend using stiff bushings. They will break in over time. For more seasoned skateboarders, medium bushings are usually the perfect choice. I would only recommend soft bushings to skaters who want to spend most of their time carving on their skateboarding. Soft bushings can make tricks difficult, and require a lot of control.
The truck height can vary. Low trucks make flip tricks easier and add some stability, but with lower trucks you will want smaller wheels. High trucks allow you to use larger wheels, which will help when skateboarding at higher speeds or long distances.
If you are a new skateboarder, I recommend using medium trucks, unless you know for sure that you want to use your skateboard for street or cruising. For street, low trucks are good and for cruising, medium or high trucks are a good choice.
For help on picking out a good brand of trucks, see the Top 10 Skateboard Trucks list.