That happens when you do stockinette stitch. You can limit the curling by knitting a few of your first stitches on every row, knit or purl. and the end of each row. You'll basically make a garter stitch border for yourself.
You can start out by knitting the first 3 or 4 rows after casting on in garter stitch. Then on your first all purl row, knit 3 stitches then purl till you have 3 left and knit those too. Flip around and just knit your whole knit row. Keep up this way until you're ready to finish and do garter stitch for your last 2 or 3 rows before binding off and you have a garter border framing your stockinette stitch. I see this most often on washcloth patterns or on swatch samples in knitting how to books.
Does that make sense?