building robots

If you are willing to spend the money, pick up something like a lego mindstorms kit. Many engineering schools use these for freshman projects. You can do the searches yourself, but you can get different types of sensors (there are some really cool ones out there!) and motors which can be programmed with C (and some sort of lego API I'd assume). These kits will give you many many options for types of robots to build which can serve various purposes, with relatively little hardware/mechanical knowledge required.

Even if you didn't want to use a lego frame, the sensors and lego documentation would probably be the easiest way to do this. Keep in mind when you are working on the program logic that making a robot move in a predictable way is actually a very hard task. Moving in a straight line exactly 5 feet, turning exactly 180 degrees... these are not trivial tasks. Plan for this ahead of time!

I would also suggest Lego Mindstorms kits, they are simply fantastic.

You know those small vacuum robots that vacuum the floor all by themselves? You can buy an interface and programming kit for them. Roomba or something.

go to radioshack. In the electronics section, they have little books with all kinds of practice projects for making stuff out of radioshack parts that can get you started in learning how to engineer that sort of thing. They also have kits to teach you about soldering together circuitry.

Then you can eventually buy something like this. This is basically a chip that you can program in BASIC. Then, you wire it to motors and whatnot to control your robot.

There are a lot of Basic Stamp robot kits -- see if you can look for them.

One such kit is the Boe-Bot, which is really cheap now (for a limited time).