How do you mute a string or strings?

I assume you're talking about holding the string down to change the note and not about putting a plucking finger on before plucking.

First of all, some terms:
The neck of the guitar is the part with all the little metal bars on it.
The little metal bars are called frets.
The face of the neck, that all the frets are attached to is called the fretboard.
When I refer to the area behind a fret, I mean the side of the fret closer to the head of the guitar (the part with the tuners).
The act of pushing a string down behind a fret is called fretting a note.

Make sure your guitar is in the proper position. Set it on your right leg (assuming you're playing a right handed guitar -- probably the case) with the neck extending to your left. The neck should be roughly parallel to the ground, but tilted up a little. The fretboard should face away from you. Don't tilt the guitar back to see the face of the fretboard, you'll cramp your neck and your back leaning over it like that. Naturally you'll have to lean forward a bit at first too see what you're doing, but you'll quickly learn to play by feel.

You want to put your fingertip on the string right behind the fret, as close to it as possible. The closer you are, the less force it will take to fret the note. If notes buzz when you play them, first make sure you aren't too far from the fret. Don't press the string down yet. Make sure your finger is arched so that it's nearly perpindicular to the fretboard at its tip. Firm your finger, then relax your arm. The weight of your arm should pull down and back when you let it go, thus pressing the string down. It will take a little practice to fret the note rather than just letting your arm fall to your side, but that's how it's done. I sonetimes pull back from the shoulder a tiny bit when I need more force. If you squeeze with your thumb you'll cramp your hand. Most of the time, your thumb doesn't need to do anything.

How's that?

i mean to mute a string in tab it looks like this:

-----------0----------------------------
-----------0----------------------------
-----------5----------------------------
-----------0----------------------------
-----------x----------------------------
-----------5----------------------------
and the x means to mute the string how do i do this

The fingers you use depend on what's before or after it - let's assume you're in third position. You put your third finger, the ring finger, on the sixth string, and the fourth finger, the pinkie, on the third string. Then you lean the third finger so it touches the fifth string - that deadens it. You should still have the 1st, 2nd, and 4th strings ringing open.

You could also use one of the free fingers - in this case the index and middle - to lightly touch the fifth string, but I find that technique more difficult, paticularly at performance speeds. A bit of practice will have you leaning your fingers properly in no time.

Right hand muting isn't practical for a situation like this - there's no easy way to dampen just one string. The only right hand option you'd have would be fingerpicking - the thumb would hit the 6th string, and you'd pluck the other four strings, either using fingers 1,2,3,4 or just 1,2,3, and letting the index finger strum across the 3rd and 4th strings.