Bleed the bakes of the crap fluid .
Make sure the caliper is sitting over the disc as much as possible without making contact with anything other than the pads.
To open the pads wider to accept the disc undo the bleed nipple and push them apart (preferably with a pad seperator)
if you have just forced the calliper over the disc i pressume you have not read the tutorial on how to set youre bike up correctly