I've done an Instructable before on using bleach to create this effect (click here to view it)
, but the process is different if you're using paint. One great thing about using paint instead of bleach, is that the finished product is a lot more predictable! Bleach can leave behind some surprising results, like turning navy blue into orange. It's easier to see what you're going to get by picking out a color of shirt and a color of paint that you like.
You'll want to start with a white or light-colored shirt and darker-colored fabric paint. It's much easier to use dark paint to cover up a light fabric, than to use light paint to cover a dark fabric. Also, avoid any fabric paints that have glitter in them. The glitter will clog your spray bottle! You can use only one color, like I have done, or let the first color dry and start the whole process all over again to add a second or even third color to your shirt.
Also, you might be thinking, "Hey! I'll just get a can of spray paint from the hardware store!" Well... you can
, but the shirt will stiffen everywhere the paint touches it. It won't be nearly as pleasant to wear, as it will if you get paint that's made specifically for fabric. Most craft stores have it, and it's not expensive. So, I'd recommend that you spring for the good stuff!