Use the arrow keys or the letter to select the various options.
For Example...If you want to send a message, choose "C" for compose.

The red text describes what to type where.
To check mail that has been sent to you, press "I" for Index.
Anytime you want to return to the Main Menu after you have sent your message, just press "M".

If you found this confusing and need more detailed information, look at the On-Line Pine Tutorial!
You might want to download a copy of the Pine User's Guide!