Alright guys I need some help here with itunes and my music files

Basically, I've had two apple accounts in the past like 4 years. One day I connect my Ipod touch to my computer and downloaded an update from Itunes, not knowing that it erases all your content. I didnt have my music files backed up either, so I ended up losing over $300 worth of music and apps. Ever since then I never updated my touch, from the fear of losing my music. Now that I have my own apple account, I have music files from the first account and my account. My question is how can I back both of these groups of files up and update my ipod without losing any of this content!? I would really appreciate a step by step process etc. Thanks guys for helping me out!

-Colin Wagner