Fresh installed windows on an SSD, whats the best way to get my stuff back from my HDD?

I recently bought a 250gb SSD. My 1TB hard drive is almost full so I couldn't do a full transfer across, additionally my computer was pretty clogged up with programs I didn't need so I decided to do a fresh install of windows on my new SSD.

I unplugged the HDD, installed windows on my SSD then plugged the HDD back in. Everything worked fine but I don't know much abut this stuff so I'm not sure what to do next.

I would like to transfer stuff like my Steam games, graphics card drivers (or should I just reinstall those?) and a variety of useful programs I have on my HDD.

Do I just drag and drop them across? Also, what files should I now delete on the hard drive? I've heard you should delete the windows files, what are these?