Usually when you fit new pads you need to push the pistons back into the caliper a little due to the new pads not being worn. This said, it is surprising that the new pads fitted easily.
It does sound like a fluid issue. You will need to bleed the brake to get any air out and get the pads close to the disk. not too hard to do mut you will need a bleed hose (a plastic pipe), a container for the old fluid, a spanner for the bleed nipple and some brake fluid. You may need a screwdriver if the rear brake fluid reservoir has a cap that has a screw down fitting.
Be careful not to put too much pressure on the bleed nipple as they can shear off and make sure you have the correct brake fluid (DOT rating).
Loosen the nipple, fit the bleed hose, open the fluid reservoir, fill with fluid, open the nipple, push down the brake pedal and tighten the nipple with the pedal still down. Do this a few times keeping an eye on the fluid reservoir and topping up when necessary. The brake should start to get some bite and when it does give the pedal a couple of pumps with the nipple closed. Tighten the nipple up properly, check the reservoir is full to the top line and replace the reservoir cap. Test the brake. If is is fine then the brakes are done and you then need to check that the brake light switch is turning on at the correctly and adjust is necessarily.