A melody is a series of notes, usually within a particular key (scale) arranged in a particular and hopefully pleasing sequence. Is it easy? Not for most people. Good songwriters, musicians, arrangers, etc. have a more natural facility for it, though.

Staying within a particular key/scale, you should be able to, if only by trial and error, find the melody for any song that you're working with, tabs or no tabs. If you have a good ear, this will be pretty easy. If you don't, it'll be harder, but still doable.