how do i restore my phone back to the original rom

[stewy321]

i have no idea what im doin. it would be really helpful if someone can help me. i need to return my phone back to sprint, my friend customised it and cracked it now i need to change it back for my warranty to work. im a complete noob so i need to know every lil step that is needed. i understand that i also have to relock it. so that would be helpful as well. thanks guys