>>105346If your chosen nickname is already registered but currently not in use, you can change it with /nick new_nick without doing the whole network registration again.
Once you have registered, you can pick "Identify to NickServ" on the connect page. This is recommended if you want to automatically join channels like #4chan that require registered nicknames. Proper IRC clients have similar options.
The mods have summarized the whole process in the /qa/ sticky as well:
>>>/qa/1690512 