Hi, and welcome to the SOR forums!
Well, to use the usb port, you'll need knowledge of the usb specs. A better idea would be the parallel port (simplest electronics) or the serial port (some electronics will be involved). Using usb, you either need to know the specs, or get a usb-serial converter and do the same as if using a serial port.
To make any kind of autonomous system, you need three things. Acuators, sensors and software. All these need to be somehow linked, so you will need knowledge of electronics, mechanics (hey, you can't call a motor on a table a robot). You already have the software part, so, the next logical step, in my view, is to choose a microcontroller. These guys are computers on a chip, and will serve you to communicate with the "big" computer, or can be the computer by itself (you can write code for them in the language of your choice).
In parallel, you'd need to get the grasp of basic electronics: get a breadboard, some leds, some resistors (220ohm, 1k, 10k), some capacitors (small values pf/nf range will do) and a bunch of transistors (everything low power is ok, bc547 is a ubiquitos one) - make sure they are npn, since these are mostly spread in internet schematics.
Also, get your microcontroller. I'd recommend AVR, stay away from PICs. They are nice, but they will limit you. I discovered this after a year and half of working with them. The code you will write for the microcontroller will have to be downloaded somehow into it (think of it as a DVD-WR/DVD analogy. you can't write DVDs if you don't have a DVD writer). The device that downloads the code into the AVR is called a programmer. Common designs are available, using either the parallel, serial or usb port.
Last, you will need to get some soldering skills. Since every robot is a mixup of electronics, you need to connect those electronics somehow. You can also build the whole thing on a breadboard (something that allows you to stick/take out components from it, without any soldering).
Also, heck out the tutorials on the main page, they have a lot of pointers.