The free (I love free) Fender Tuner App is a really easy way to help tune your guitar. I really like the Auto mode which automatically determines which string you're trying to tune. It also has a manual mode where you select which string you want to tune. As shown below, it tells you if you're too flat or too sharp, and when you're spot on, the circle turns green.
A standard six-string guitar is tuned, low to high, to these frequencies.
E: 82 Hz; A: 110 Hz; D: 147 Hz; G: 196 H; B: 247 Hz; E: 330 Hz
Links to the apps in the Google Play and Apple App stores are below.