Top left : This one starts with a "proper-sized" shirt, you cut it off however high or low you want the waistline, and sew fabric onto the bottom. You don't need to limit yourself to plain-color shirts, either.
It's pretty much the same as the pattern for the pencil-drawing picture, to the left, but this one uses a pre-ruffled knit fabric. I spotted some of this at Hobby Lobby earlier this month, it's likely at most fabric stores. Or check at RuffleFabric.com (pricy but lots of options).
Top middle : Or leave the t-shirt long and add layers of ruffles yourself.
Top right : This is the kind of T-shirt dress I remember my mom making for my little sisters: you cut a piece of fabric about twice as wide as the bottom of the shirt, hem it along the bottom, gather it at the top, and sew it to the bottom of the shirt. If you're nervous about gathering or just want another option, you could cut a circle skirt.
Bottom : make it a patchwork skirt, with a big bow in back.