With overwatering, the leaves are usually both limp and yellow. With underwatering, the leaves are usually just limp. Try sticking your finger in the soil. Is it moist?

Underwatering usually isn't a problem for first-time growers, but if you're just sprinkling a little on, that could be it. Have you watered until it starts dripping out the bottom yet?

One nice thing about underwatering is that when you do water, the plant perks up right away.