PC upgrade help

I have a PC, couple years old. I know I need to upgrade the HD. Probably wouldn't hurt to do a new video card also.

First, how do I know how many ports my current HD is using? Like do I need to just get another 500 G SSD and stick it in there, or would I need to replace what's there with 1 TB? I'm guessing I only have one because all I have is a C:, but I don't know if that means there's a second port I can just stick another HD onto. How do I figure out what ports are open? And are they all pretty much standard ports, or do I need to find out what type of port mine supports?

Same with a video card. Are they all standard ports and if I find one I like at a good price, it should just replace? Or would I need to make sure it's the right port?

I just don't want to buy a bunch of stuff then find out that they don't fit.

This is what I currently have.

https://www.newegg.com/skytech-st-shadow-0003-ne/p/N82E16883289038?Item=N82E16883289038
Thanks in advance.