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.

Now, if you took it off and laid it flat, and drew lines to connect all the pins, you'd have something that looks like this.
So do it. Make yours look like mine.

Once you've got the lines drawn, remove the pins.

If you draw lines and cut here, you have a standard girly tee pattern. Hoorah!

Cut here, and you've got a perfect tube top pattern!

You'll probably want to take these pieces and transfer them to paper. It stores more easily and is easier to cut around than hunks of t-shirt.
Any kind of paper will do. Some people like tissue paper, but I think it rips too easily. Paper bags will work, but they're a little thick. I prefer the brown packing paper that comes in a lot of shipping boxes, or blank newsprint. Wrapping paper also works well and looks pretty. My favorite stuff is tyvek. It's what they make the UPS and post office envelopes out of, and it is IMPOSSIBLE to rip.

That's it. It's probably not how the professionals do it, but it should give you somewhere to start if you're dead set on jumping head-first into pattern making. If you were looking for something more complicated, then you'll have to figure it out yourself!