How do I reset what programs open with? (defaults)

hello, i just got a mac computer a few months ago. i'm a big music lover, so when i noticed downloads going to the download folder, i changed the defaults to open all similar files with itunes. unfortunately, now everything opens with itunes. any application i download (skype for example) opens with itunes or text edit. is there any way to reset the programs to open with what they're supposed to? i really hope that it's not hard to understand what i'm talking about and i appreciate any help you can give me. thank you so much, Olivia