Well said Ross.

There are no accepted ways to put chords behind a tune, because ALL THE TUNES ARE DIFFERENT. It's a matter of learning which chords are generally associated with particular keys and learning to use your ears.

My best advice is not to play too loud until you know what you're doing and then don't play too loud - particulalrly if there are other backing instruments in a session. Many backing instruments in a session can work, but only if they are all listening to the tune (and each other).

The reason it is known as backing is that it should be in the background - audible but not 'sticking out'. The odd 'pagan' chord, as a friend of mine would describe them, can liven up an accompaniment but over use of them is jarring. Simple is usually better. If you find that a particular chord works well in a tune use it ocasionally. Remember that arrangements that have been put together by a band may only sound good in their particular arrangement of the tune.

If your ego doesn't let you stay in the background, you're probably playing the wrong instrument!