The F chord on a guitar is one of the most used, yet, least understood chords for beginners. It is also feared by novices because it introduces a "barre" across two strings, a difficult technique to master at first, which leads to unintentional mutes and unpleasant dissonances. With practice, however, it can be perfected and will prove to be an essential element in any guitarist's repertoire.
Place the ring finger of your left hand (unless you are left-handed, in which case all steps should be performed with your right hand) on the D sting (third from the top when looking down from a playing position), third fret.
Place the middle finger on the G string (fourth from top), second fret.
Place your index finger across both the B and E strings (fifth and sixth from the top) on the first fret. This process of holding a single finger across multiple strings is called a "barre" and is a frequently used technique in guitar playing.
Apply firm even pressure across all strings that you have fretted and strum only the third, fourth, fifth and sixth strings. Do not play the top two strings. You will probably notice that the sound is less than ideal because it takes some practice to learn how much pressure to apply and how to hold both barred strings down without muting one or the other, but continued practice will lead to improved technique.
Beginner guitar players will notice that their fingers will begin to hurt after a short period for playing as a consequence of the strings biting into the flesh of the fingertips. This is a natural phenomenon and will gradually ease as continued practices build thicker layers of skin, called callouses, on your fingertips.