You are going to get a lot of different views here and the one that will most propably come to the forefront many times is this... Play what sounds good...
If you are just starting out, get a song you know and play the chords/notes in a different timming placing emphasis on different notes than the original. You might event change a couple of chord voicings and notes where you feel is appropriate...
Then go out and try to put the song, melodies and harmonies into a theoretical context if you wish to... This will allow you to understand the mechanics behind this "new" song of yours.
But most importantly play what sounds nice to you...
