There are two main things to consider when buying an acoustic guitar for a beginner: ease of play and sound.
The two main factors that make an acoustic guitar easier to play are its form and its action. You may have trouble moving your arm over the body of a dreadnought or jumbo acoustic guitar. If this is the case, you should try a form of "folk" or "living room" which is smaller and therefore easier to play. The action refers to the distance between the string and the frets. If the action is weak, the ropes will not have to travel so far, which will ease the pressure. Sound is also an important factor to consider. Make sure you are satisfied with the sound produced by the guitar, you will probably want to play it more often!