Simplest way is to take the old hard drive out and take it there, they can tell you what you need
Take a screenshot of the data plate and ask someone there for help.
Also strongly consider SSD (solid state drive) over a spinning hard drive. Prices for SSD have dropped, and the speed is incredible.
Most important is interface. If it newer than say 7 years it should be sata. If older it will be ide. If you get the right interface, any will work. If it is a laptop, then physical size matters. Older operating systems say xp and older, there is size limits gigabyte, megabytes, etc.
Thanks! Your answer is awaiting moderation.