What will happen if i reset my phone?

You won't lose your contacts or anything else that is saved to your phone. Resetting just puts everything back on default settings. You will have to change your personal settings back to what they were, but you will not lose anything.

Resetting your phone removes personal settings (wallpapers, ringtones, etc) and will remove all your applications. App data may not be removed if you have allowed apps to be backed up to Google Play. So the applications themselves will be removed, but you can get back all, if not most, of your data when you've reinstalled them from the Play Store. There's no need to move all your pictures and music as personal documents such as those will be retained in the memory card, unless you opted for a complete factory reset.

However, note that your call logs, SMSes, messenger chats (such as WhatsApp, LINE, etc) will be deleted unless you've backed them up. All accounts, such as gmail accounts, on the phone will also be removed, so you'll have to login again.

Hope this helps.