Chords
The cords illustrated below, on the bottom staff, are the most typical cords in the key of C, which is the simplest key to play in. The notes on the top staff are typical of what notes you might be playing at the same time as playing the cord.
I usually go about it the other way around. If I'm playing a C note in the melody, I will typically form a C cord with the left. If I'm playing a D with the right hand, I'll typically form a G cord with the left. There's no hard and fast rules, play what sounds good. (The small grids above the staff are for guitarists.)