how can i make my epiphone les paul standard sound more like AC/DC. could you give me suggestions as to which amp i should buy
well...it's not so much the gear as how you play. Hit the strings hard, and use heavy strings (.13s) for the Malcolm rhythm tone and use light strings (.09s or .10s) for the Angus tone. It's mostly attitude.

Anyway, Malcolm uses a Gretsch that he's taken all put the bridge pup out of, strung with .13s. Angus uses a Gibson SG strung with .09s or .10s, not sure which.I'm pretty sure both use Marshall Plexis.

That's pretty much it, not really a secret...
you guitar is going to have less to do with achieving that tone than your amp. The easiest way to get it is to crank a tube amp. I don't know your price range I can't really suggest which amp to get.
Short of that, an OD pedal might do it for you.
"There he goes. One of God's own prototypes. Some kind of high powered mutant never even considered for mass production. Too weird to live, and too rare to die."-Duke
you have humbuckers, so now just turn your amp up. little to no preamp saturation if you can help it, and turn the power amp up as far as you can stand. assuming you have a tube amp of course (preferably with a brittish voice). thats it, thats ac/dc tone.
Turn your treble up, set the guitar to the bridge pickup. Do nothing but play the same three chords, and throw a pentatonic-scale solo in there. Sell millions of albums.