3 Answers

Is it a new string?  Older strings get loose and wear out.  So, if it is older I would change all of the strings (unless you have replaced one or two in the last month).  Once the new stings are on retune them all..and check the tuning often- at least everyday before you play.  I work with two bands and sometimes they check their tuning after an hour of heavy strumming.  Also, make sure you srting it correctly and that it is tight before you play.  You can have the parts checked by anyone who knows a lot about guitars- a freind could do this for free.

If your guitar is used, it depends when it was last played. If it was stored for some time with the strings on and not played, the neck could have bowed. Replace all the strings with new ones and have it tuned professionaly.

New guitar strings tend to get out of tune right after their are put on a guitar. It takes few days of tuning for them to adjust and reach the proper stretch.