Measure the mattress, figure out how far down you want the quilt to hang all the way around. Do you want the top of the quilt to wrap around the pillows? This will determine the finished size of the quilt.
Do you want borders? How many, and how wide each? Are you wanting sashes? Then you figure out the size of your blocks.
Or you can start out with say 10" blocks and figure it all out from there... graph paper can help a lot with the planning stages.
Sometimes it is easier for a beginner to buy a pattern where all of this is figured out for you :D:D:D