if I reset all settings what happens ??, if I reset all settings what happens ??

Exactly, what ID are you talking about?

if i reset all settings will my messages go away???

If you are talking about your Apple ID all you need to do is go to settings, Itunes and app store, tap your finger on the apple ID shown there and log out and back in with your apple ID you want to use...

If you are talking about your icloud account, now that I am on ios7 I can not change my username (email address) for my icloud account. Or so I thought until I found this page - https://discussions.apple.com/thread/3418775?start=15&tstart=0