Start by opening a direct chat with NickServ. It's generally @_<network>_NickServ:homeserver.example.org, though some networks omit the leading _ like freenode: @freenode_NickServ:matrix.org. You can figure out the network prefix from Bridged IRC networks.

Then you can register/identify normally depending on the syntax of your network which you can generally check by saying "help register" or "help identify".

For example freenode uses the syntaxes:

    identify [<nickname>] <password>
        where nickname is optional assuming you are identifying for your current nickname
    register <password> <email>