Before you start playing, you need to tune your guitar.
You’ll need a guitar tuner to tune a guitar. Some people learn to tune ‘by ear’, but this is more advanced!
There are many free apps available that can listen to each string and enable you to tune your guitar. I prefer to use a clip on tuner like the TC Electronic clip on tuner in this video to get the guitar in tune as quickly as possible.
5th fret tuning trick - tune your guitar by ear!
The notes at 5th fret can be used to check the tuning of your instrument, providing you know the first note is an E.
This is a very handy trick if you suspect that your guitar might have gone out of tune and want to check quickly! N.B. The only note that isn't at 5th fret is the B, which is at 4th simply because of how a guitar is tuned.
If you need more help and want to make the fastest possible progress as a beginner, you may wish to checkout the more detailed lessons in my online Beginners Course, where the chords, changes, strumming etc are broken down into separate videos.