Hi Isabel...

Bassic has given you a good starting place...to fine tune, find the octaves in simple chords like Am and Em and listen for resonance. A lot of guitar players now use electronic tuners. For some reason they always screw me up. But a good quality pitch pipe for the guitar is about $6 US in any music store. As long as the strings are tuned relative to each other the chords will play true. So some people actually tune up or down to fit their voice. On the other hand, some guitars really sing only when tuned to concert pitch (starting with that E that Bassic mentioned).

If your guitar is one that you value it might also be wise to keep it in a case rather than leaving it out. It's safer, and more likely to stay in tune, at least that has been my experience.