You haven't said which IRC client you're using, so I'll be using Freenode's Kiwi chat as an example.
Most IRC clients will have tabs for each channel you're in and each private messaging conversation you have with a user. They will also generally have a Status channel. This channel gives information about the server and your connection to said server.
To register with Freenode's services, you need to message NickServ, which is essentially a bot. The
/msg command is used to start a private message conversation with a user or, in this case, a bot. The command can be entered into any tab associated with the network you're on.
If you're using the Kiwi chat, there is only one network: Freenode.
You can write the command
/msg NickServ REGISTER password firstname.lastname@example.org (where you replace
password with your password and
email@example.com with your email address) into any of the tabs you see in the image above.
This will create a new tab where the NickServ bot will respond to you. You can then issue commands to the bot without the
/msg NickServ portion if you enter the commands into that specific private message tab.
Once you're registered, you will need to
IDENTIFY with NickServ every time you log in. You will do this by either using
/msg NickServ IDENTIFY password:
IDENTIFY identifies you with services so that you
can perform general maintenance and commands that
require you to be logged in.
Syntax: IDENTIFY <password>
You can also identify for another nick than you
are currently using.
Syntax: IDENTIFY <nick> <password>
/msg NickServ IDENTIFY foo
/msg NickServ IDENTIFY jilles foo
Or you can enter your password on the login page: