The caliper bolts can be tight so be prepared to use a wrench with plenty of leverage.
The pistons could slide back easily or they could be tight aswell, sometimes I use a block of wood and a crowbar to lever them back into the caliper or a G clamp. Careful that you don't damage the piston seals when doing this.
You'll find it easier to unbolt and remove the caliper, remove the brake disc and then fit the caliper back in place temporarily with the bolts so it's held steady to make it easier to push the pistons back in.
Sometimes the brake disc rusts onto the hub making it difficult to remove, on some discs there'll be a couple of threaded holes which you can fit in a couple of bolts (M8 AFAIR ), tighten them evenly and they'll push the disc from the hub.