almost certain you have the two piece hub/rotor, especially if you have 11.25" diameter vented discs.

Remove the caliper/holder (more work than just pulling the clamper to put in new brake pads) and coat-hanger wire it up out of the way.

Unscrew the locator pin on the hub...it holds/positions the rotor. The rotor should come right off...if not, rap it sharply with a brass/dead blow hammer 180 degrees twice and it should just about fall off.

Some people don't like to shave discs, preferring to replace them with new ones, arguing that thermal reservoir is reduced when turned, etc., residual stresses, etc., etc. I'd say as long as you get a good turn job with nondirectional finish and stay thicker/at minimum thickness, you should be ok.

If that doesn't do it, jsut replace the discs. You do have to replace both sides at once.