scared to sync

I am very computer illiterate...

I had to get a new pc laptop...when I downloaded itunes onto it and authorized it said I have authorized 3 computers. My itunes account has jumped around with me. The last time I attempted to do something with my ipod classic (connecting to our new imac- which I no longer have), I wiped out 971 photos and cried for days! Now I am scared everytime it says sync. How do I get the music off of my ipod classic and onto itunes on my computer without losing anything?

I appreciate step by step, dumbed down answers please!:)