iTunes problems! Please help!?

Alright, so what happend was….
I've had my iPod for a few years now and I have all of my songs on one computer. Well now I have a new computer that I recieved for Christmas, and when I plugged my iPod into the new computer, the songs that I put on myself didn't upload, only the songs that I bought/downloaded! My old computer is still up and runner in my guest room, but it's really slow and I need to figure out how to get my songs on the new computer. HELP!!!

ps. Please make the instructions step-by-step and easy to follow!

Please help me! This is very important because I can't upload any new songs now! HELP ME OUT!