Depending on how much clearance your frame and brakes have to the wheels will determine how agressive of a tire you can mount. You shouldn't have much problem though.
Check out
these tires for suggestions of good tires to mount to your Trek. Something in the $12-20ea range is a good price to get a tire that will hold up for you. You'll want to get a 700c tire, width of 28-35mm. The wider will provide more grip, but higher rolling resistance (making it harder to keep the speed up).
A couple photos of your bike or a link to one that looks simular will help us determine what will work on your Trek. A rigid fork is most likely possible, but probably not worth the cost.