There is a tension screw in the body of an acoustic guitar which is used for bridge adjustment. Tightening this scre will raise the bridge while loosening this screw will lower the bridge. that being said, adjusting the bridge can be like adjusting spokes on a bike. Once you star, there is no going back. I strongly reccomend having a professional take a look at your guitar and allow him.her to perform the adjustment.If you don't know what you're doing, you're going to end up at the guitar shop anyway. Good luck!
Depends on what kind of bridge and what kind of adjustment. Assuming you mean the string length adjustment on an electric guitar with individual adjusters, compare the pitches of the natural harmonic at the twelfth fret with the fretted note. If the fretted note is flat to the harmonic, adjust the string length shorter. Once the two notes are in unison, do the same to each string in turn.
If you mean the bridge height, that's more complicated. You should start by adjusting the neck to proper relief, then adjust the bridge to optimal height, which for most people means low enough for ease of playing but not so low as to cause buzzing or rattling against the frets. This height will vary widely depending on the type of guitar and type and gauge of strings.