If it's already installed in your distro then type "minicom" at the command line. If it's not installed, well, you need to install it, don't you? Check to see if your distro has it available as a package to install.
Using it...I actually don't remember that well. It's basically a clone of a DOS program called Telix, I remember that. "man minicom" at the command line should provide technical documentation. There's also a help option in the program itself.