Your guitar must be in tune. All guitarists must learn how to tune a guitar. There are two ways of doing this. 1) Use a guitar tuner. Guitar tuners can be physical devices or apps. The great news is that if you search "guitar tuner" in the app store or google play, you will find a free app. The one I use is called "Da Tuner". 2) You can tune your guitar by ear. This is trickier but can be useful if you need to tune your guitar and do not have a guitar tuner. In the following videos, I will go through both ways of tuning your guitar.
Most guitarists tune their guitar to standard tuning. Standard tuning from the thickest to the thinnest string is E A D G B E