It is important to have the correct set of strings fitted to your guitar, especially if you are a beginner. Until you build enough strength in your hands to fret the chords cleanly, light gauge or low tension strings are recommended.
A reputable music store which sells guitar strings should be able to assist with this. It is important to change your strings regularly, as old strings go out of tune easily and are more difficult to keep in tune. In time, you may wish to experiment with heavier string gauges as they generally produce a stronger tone.