imported post
Not a lot of info to go on there, but the basics would be to pull all the fuses (except main one) and turn the ignition on. If the main fuse doesn't blow then one of the other fused circuits is to blame. Put in the fuses one at a time until you get a blow and then you will know where to look further.
Other questions you need to ask, like was the bike running before you bought it? Does the main fuse only blow when you press the starter button (if so, dodgy starter or shorted feed wire to the solenoid). Try running a heavy wire direct from the battery to the starter terminal (just tip the wire to the terminal for a second) and see if the starter works.
Like I said, you didn't give a lot of info and I could go on until I run out of space in the message box. Try the suggestions above and get back to us later.