Generally, you start out with about 8 sts, double the stitches on the first round, knit a round plain, then add 8 sts every other round. You can use YOs or any increase you'd like.
A square is a little harder to do, but is possible. Again start out the same as in a circle, but at the 3rd or 4th inc round, do double incs on the 4 'corners'. That would be something like - k7, YO, K YO (or K YO K in one stitch) and repeat around. By making 2 incs at each spot instead of evenly spacing them, you'll get more of a square shape.
Take some leftover yarn and play with it, see what you get.