First let me say that pattern making is a pretty advanced skill. It's not really something I recommend you start out with immediately, mostly because you will probably end up really frustrated. It's hard. You have to take 2 dimensional (i.e. FLAT) pattern pieces and make them fit your curvy, lumpy body.
I recommend starting out with storebought patterns, because you'll get a chance to see what a pattern looks like, and what the basic pieces are shaped like.
But if my words of warning are not enough to sway you, I suppose I have no choice but to give you my crash course in making a very VERY basic pattern.
Grab yourself a t-shirt (preferably a crappy one you won't mind cutting to pieces) that fits you like a potato sack.
Put it on inside out.
Pin it so it fits. Pay special attention to the curves of your bust, waist, and hips and your armpits. You might want help from a friend at this point, because it can be kind of tricky to hold the fabric in place on yourself AND pin.
Pin it snugly, but give yourself enough wiggle room to get it off with all of the pins intact.