Just decide on a size that is a multiple of 4 or 6. Cut out squares that are one-half an inch larger than your multiple number (4-1/2" or 6-1/2"). Then, sew the squares together using 1/4" seams and voila - a charm quilt. You can have fun putting your fabrics together to make a pattern while you sew them together.
You will want to buy a few quilt supplies, like a cutting mat and rotary cutter to make whatever pattern you decide on a little easier.