The following are things you should (or ideally should) have:

• LEGO elements. LEGO is available either in bulk packages with a variety of bricks, plates, and other elements, or as sets which can be used to build the model on the box or other creations.
• Base-plates can usually be bought individually, although some come with sets. Like their name implies, they act as a base for models and help provide support. Base-plates are most available in 32x32 and 48x48, but various sizes, colors, and shapes exist.
• Mini-figures add realism to LEGO models. They are generally included with some sets, and some special sets are released which are just mini-figures and accessories. Again, consider what you want to do before purchasing.
• If you are building a Technic model, wheels, axles, and other elements are included in LEGO Technic sets. An electric motor can be purchased separately to add motion to the model.
• If necessary, sets and parts can be purchased from the LEGO company's Shop-at-Home catalog, or from various web sites that buy and sell LEGO.