Always begin by visiting a paint store and picking up as many paint chips as you wish. Then narrow your choices down to three or so, and buy a quart of each. Apply wide stripes of the colors side by side on a sheet of cardboard and tape the cardboard to a wall in the room you'll be painting.
Leave it there for 24 hours, checking the colors throughout the period to see how they look in the changing light. At the end of the 24 hours, you will probably have decided on one color as the one you want for the walls. Yellow goes well with a sky-blue hue for the ceiling (using the same techniques as for the walls) and a brilliant white for the trim.
Also start looking at decorating magazines and tearing out pages when something appeals to you. File the pages in a folder, and after a week or two, spread the pages out and look at all of them to get an idea of the direction in which your taste runs.
Enjoy painting ka-Aplus!