What do you do if you murdered someone?

Murder/homicide implies pre-meditation (i.e.: pre-planning), unless you actually mean Manslaugher (an un-premediated or acidental killing). All in all, generally speaking, the law will catch up with you eventually. If you did kill somone without the intention of doing so, it really is best to just turn yourself in, get a good attorney, and get it all over with.
Murder/homicide/manslaugher is one of the most difficult of all offenses to get away with. There is no statute of limitations on this offense and the odds are that you will be caught eventually.