Replying to IPod Help

I'm having trouble getting music from my iPod that I've purchased directly on it to my computer. So I got a new computer and I downloaded iTunes onto it and signed into my account, and it has a lot of my new songs, but it doesn't have all of the ones from my iPod and it wont sync them. Whenever I go to 'My Devices" and select it they don't show any of the iPod only songs. I have looked a lot on the internet and everything says the same thing. I think I need to update my iPod but if I do it says it will erase my songs on there. +K of course for help thanks everyone.