Using your left hand, play C with your pinkie and E with your third finger. Move your fingers up one key and play D with your pinkie and F with your third finger on the piano. Continue moving your fingers up the keyboard and playing two white keys. You now have learned how to play two-note chords.
To build a triad from a two-note chord, just add one more note. With your left hand, play C and E again, then add a G by playing it with your thumb. Move your hand up one note and play D and F, then add an A by playing it with your thumb. Continue moving your fingers up the keyboard and playing the triads.
To build a seventh chord from a triad, just add one more note. Play C, E and G, as above, and add the fourth note, the B, to form the basis of a C7. To play an actual C7, flatten the B (black key above the B). Continue by moving your hand up to the D key and play every alternate white key to create your triad, and then skip up one more white key and play the fourth note, the C. Sharpen your F and you have built a D7. Move your hand up again to play an E, G, B and skip up a key and add a D. Sharpen your G and you have built an E7.
Another way to add richness is to play a four-note chord that is a filled-in octave. Play C, E and G, as before, and add another C, which will be two white keys up from the G. Play a D, F and A, then add another D, which is two white keys up from the A. The first and fourth notes will be the same note, but the fourth note will be eight notes higher.
Using these patterns, continue playing two-, three- and four-note chords and adding the melody with your right hand. In time, you will learn how to improvise by adding and subtracting notes to make your playing sound best.