This question is rather open-ended. Depends on what you're building, what you like, and your budget.
1. X-acto knife with some #11 blades. Get some chisel tips, too, if you're going to be using photoetch (PE) parts, or have parts that are snuggled up next to the sprue tree.
2. Xuron shears are probably the best for cutting plastic from the sprue. A set of mini diagonal cutters (available from Sears) is good if budget is an issue.
3. Some sanding sticks or blocks. You can get these cheap from places that sell women's beauty supplies. Look in the nail section. Also give consideration to a Flex-i-file set for curves (like barrels, fuselages, etc.). Grits should range anywhere from 320 (coarse) to 600 (fine) for basic modeling. If you're doing cars or aircraft canopies, then look at grits in the 1200-2000 range. Flex-i-file also makes sanding sticks in various grits.
4. Get some good forceps (tweezers). Your fingers can only grasp parts that are so big.
I have several varieties, but the pointed ones seem to find themselves between my fingers most often.
5. Glues. Here it boils down to preference. Testor's liquid cement and Tamiya liquid cement are excellent for joining styrene to styrene. You may also want look at Tenax. Ambroid is a little too toxic for my tastes. In addition, you may also want to investigate super glues, also called CA here on the forum. They're good for filling seams and joining metals to plastic, as well as plastic to plastic. Zap-a-Gap seems to be one of the favorites. You can get it in thin and thick variants. Experimentation is the only way to determine what is right for you.
6. Get a brush to apply glues. Natural hair, round, 1/0 or smaller. Tamiya cement comes with a small brush attached to the cap. It's great. The one that comes with Testor's liquid cement is too big.
7. Putty. This one starts arguments and elicits groans. I personally like Tamiya putty and DuPont red lacquer putty--available at stores carrying auto paint. Others like the green and white putties marketed by Squadron (www.squadron.com). Putty is necessary for covering sink holes, ejector pin marks, glue seams and other flaw in the plastic.
8. Get a pin vise and some miniature drills (#68-80 or so). These are great for--well--drilling holes (gun barrels, headlights, etc.)
The list can go on and on. I would encourage you to visit www.micromark.com
They are miniature-specific in what they sell. Ask 'em for a catalog. You'll find stuff you can and can't live without. Also, visit your local hobby shop (LHS). They'll appreciate the business.
Gip Winecoff
PS After you're done with all the above stuff, you're going to need a Dremel tool, a drill press, band saw, table saw, miter saw, small engine lathe, vertical mill.....
GW