How and where to learn DOS commands?

Answer 1

there are a lot of info on the internet.

Just google for "DOS command"

Answer 2

1) Open p a command prompt (more on this later)
2) type HELP - this will give you a list of all the DOS commands
3) select any one of them and type HELP XXXX where XXXX is the one you want more detail on. It will then tell you the details.

Command prompt? If you don't see it listed on the start menu, go to Run, and enter CMD - it will open up the DOS window, a.k.a the "Command shell"