First check below how to reset your password.
http://windows.microsoft.com/en-us/w...ord-forgot-faq
But if you really want to use another account.
First of all you will loose all data, pictures etc on your phone so back them up on a PC or if they are on your SD card get your SD card out.
Be sure your phone has enough battery or is connected to a charger
Go to SETTINGS > go to ABOUT >
Tap "reset my phone"
And follow the instructions on the screen.
http://www.windowsphone.com/en-us/ho...reset-my-phone
And if you want to know how to make a Microsoft Account.
http://windows.microsoft.com/en-US/w...te-account-how