How do I use OBD scan tools?

I have no idea how to use OBD scan tools

I'm planning on buying a used car but I want to know how to use a scan tool(before i buy one) so i could check for problems. I'd really appreciate some help.

No. 1-1

You will want to use the drive cycle monitor that good scan tools have (cheap ones won't have that feature). It will tell you if the car's computer was reset. You still want to pay a mechanic to do a final check on the car before you buy.