Just buy a GPU and put inside your PC (assuming you aren't using a laptop or Mac) and go from there. If you like what you are doing then buy more GPUs and you'll need risers for more than > 2 GPUs.
Thats the trouble i use a laptop and my old tower pc is ancient.
what specs are the tower pc?
Its 10+ years old, its got an amd athlon 2.8ghz(?) single core cpu, no idea on graphics card. The psu is 500w and before the 80+ ratings came into affect.
If you can find, get RX 470, 480, 570 or 580. AMD is however insanely hard to find in stock. If you can't, try to get your hands on GTX 1060 or 1070. When it comes to Nvidia, I recommend GTX 1060 over all others. They hash at about 19 out of the box, 22 or so overclocked, some people have pushed them to 23-24. 1070 is at about 30-32 but costs twice as much. 1080 is not good for Ether mining and 1080Ti is too expensive and hashes about 36 overclocked. Fury X and 980Ti hash alright but draw a lot of power, not worth it unless you have free electricity (or very cheap electricity).
Your computer is old, there's a chance you will have to buy new PSU. Not sure about that, since we don't know what PSU you have and it also depends on what card you buy (6 pin, 8 pin, 6+8 pin connectors etc).
