Buildapc

Ahoy hoy! After some great suggestions and a second pair of eyes I managed to figure out my posting problem was related to the PSU, once replaced I got everything up. Hooked up my old HDD and the new SSD. When I tried to boot it didn't register me old HDD as a boot option, so I unhooked it an installed a fresh windows copy on the SSD, an was able to get it up an runnin, rebooked the old HDD and can see both now, but anything on my desktop or any program on the old one isn't really accessible. What's the best way to get my old HDD moved to the new SSD?