A used guitar is good way to start but there are a couple of things to look for when buying.
Look down the neck, it should be straight and have no twist or missing frets. It may have a slight upwards bend created by the string tension .
Check the tuning keys, they should turn freely with no rough spots and securely mounted.
Check the bridge area it should be secure and show no signs of damage.
Check the area where the neck meets the body for cracks or other damage.
Next you'll need a tuner, lots of choices out there at a reasonable price
The strings are tuned with the heaviest string tuned to an E then from there it's A, D, G
In my opinion it best to start with simple songs and stick to playing root notes. A root note is simply the bass of a chord. These can simply be played on the top two strings the E&A string.
Use the Internet look up beginners songs, pick a song you know is a good one to start with. Learn the basic patterns then sing along to get the tempo and speed.