Use :help to get help.

To exit out of help, type
:q<ENTER>

You can specify a command to get help with, too.
:help <COMMAND>

For example, if you wanted to know what "dd" does, type:
:help dd<ENTER>

You can also get help on a given subject:
:help {subject}

Example:
:help count

You can even get help with :help.
:help :help<ENTER>

The short version of the :help command is :h

To switch between the help window and the editing window, use CTRL-W CTRL-W