If you have a stem, like in the picture, then either a wrench or a socket will help you remove the stem.
Then you can take the stem down to the hardware store and try to match it up.
Most of the time, we like to replace the seat too.
If this is too much, and it is for most people, then it's "plumber time".
Don't feel bad, I don't work on my own cars anymore either.
If you could take a picture of it, it would help.