How to play gutair?

Learn how to read guitar tabs. Guitarists have their own system of music notation called guitar tablature, or guitar tabs for short. The basic idea is to look at the tab in the same way you look at your guitar; each line corresponds to a string, and each number tells you which fret to hold down when plucking that string.

Learn finger placement. The frets are the metal strips that run perpendicular to the strings. You actually press your finger down between the metal strips, not on them. For example, if you're playing the third fret, you place your finger on the string between the second and third metal strip. Place your finger as close to the fret as possible to create a good sound.
Hold the string down firmly so that it only vibrates between your finger and your strumming hand.

Do not forget that you must play on your fingertips and not on the soft part of your finger. This requires cutting your nails to avoid scratching the fretboard.

Right-handed players use their left hand for fretting and use their right hand for picking. Lefties are vice versa.

When you hold down multiple strings at once at different frets (to play chords), it can be a little tricky (especially if you have short, inflexible fingers). There are usually several different ways to position your fingers for the same chord, so research them and experiment to find which one feels most comfortable for you.

Keep in mind that every time you move from one fret to another, the resulting pitch will be half a step higher or lower (i.e. "sharp" or "flat"). This is important for if you want to eventually read and play from sheet music.

Some people find that placing the thumb in the middle of the back of the neck, not coming over the top, leads to better finger placement because it allows better reach and strength of the fingers on the frets. Ultimately, however, do what feels best for you.