ok newbs it scars if you leave it out , it shouldn't hurt, if you do what the piercer says then it wont be infected unless you decide to fuck with it alot and stick pencils and shit in it then it will be infected and hurt like hell, no you dont have to take it out to eat or sleep but if you have a ring then you have to take it out after you eat to clean it until you learn to eat with it or risk infection...if you have a stud you should still clean it every once in a while be cause it is a foreign object that is in your lip so white cells wont be happy, so yeah ive peirced my own lip (DONT DO IT) like 4 times so yeah it does scar not like OMG Visible though unless you tan alot then you will have what looks like a jizz spot on your chin( just the truth)
Before you go, make sure you know if you're allergic to sterling silver/gold/whatever (usually it's safe to just go with surgical steel - though some people are allergic to that too.) :)
Later
Ps Infection DOESNT HAPPEN TO 99% of the PEOPLE Its just newbs like ^ are the ones who get it infected...Trust me i study pierceing in my spare time which is like 24/7 (hooray for no school :)...)
and one last thing with a stud it can cause your enamel on your teeth to crack/chip/disappear...you might get in the habit of flicking your piercing so yeah DONT do that will have the same effects as a stud hitting your teeth....
If you dont believe me, do the research and talk to someone who has one and actually knows about it like the piercers