Well, I have a Epiphone sg G-400
and I've been playing it for a while
and never messed around with bridge[saddle]
but my teacher told me that my guitar gets out of tune when I play chords and he said i have to get the bridge [re]set.

Can I do this myself [succesfully] ?
or should I go to a guitar store and see if they'll do it ?
and how much will it cost ?

Thanx ahead
You can re-intonate it yourself, or have a guitar tech do it for $10.

Basically what you do is tune precisely, play an open note, then play a note on the 12th fret, if the note on the 12th fret is flat from the open note, tune the screw on the bridge to the right a little bit. If the note on the 12th fret is sharp, turn the screw to the left a small amount. Work with it until the intonation is just right, and make sure to stretch the strings a bit and re-tune every time you move the saddle, so you know the intonation is perfect.

In my opinion, your guitar teacher should have been more specific about what needs to be fixed. You go into a guitar store and tell them you need your bridge reset, and they'll probably look at you like you're a moron.
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thanx alot man
he's not really my guitar teacher, he teaches me stuff because I play at school.