Guitar help?

i'm a beginner at guitar and i really don't know too much as far as chords and such go, but i do know like the parts and other things are pretty much obvious. the guitar that i'm using is a squier strat. i got this guitar from my grandparents because it was just sitting around in their basement for who knows how long. The problem I have is that after I play it for a while or after i leave the guitar alone for a day or so, all of the strings are flat the next time i want to play it. I was wondering what the problem actually is and what i need to do about it?