i have someones apple id on my phone and i cant sign back into mine

There are several different places where you can sign in to an Apple ID:

• Settings/iCloud
• Settings/iTunes & App Stores
• Settings/iMessage
• Settings/Facetime

In addition, if you install an app that someone else has downloaded onto their computer the app will always be associated with the ID that it was downloaded with.

So which of these is the case?

Go to Setings/iTunes&App Stores. Sign out. Then sign in with his Apple ID and password. The only reason it might not let him is if the Apple ID or passcode is wrong.

What is the message he gets when he does this?

That is the iCloud Apple ID and password, not the app store Apple ID and password.

Go to Settings/iTunes & App Stores. Tap on the Apple ID. Tap on Sign Out. You should not be asked anything, It should just sign out.

It sounds like he is not going to Settings/iTunes & App Stores, but instead is going to Settings/iCloud. You must know the Apple ID and passcode to sign out of iCloud, because doing so disables Find my iPhone; this is the antitheft feature of the phone. If there is someone else's Apple ID in iCloud settings then the phone belongs to that person. Syncing to a different computer will never change the iCloud account the phone is associated with. So if this is the problem there's more to this story than you have told us.