Are you following the
Beginners Guide on the Arch Wiki? It walks you through the complete installation from starting with the Live CD to a full DE if required. The Arch Wiki is very good and you can usually find everything you need on it (apologies for the RTM type post - it's not meant that way).
To answer your question, section 3.4 of the Beginners Guide linked above gives some details of adding a user. An easier method than useradd (in my opinion) is to use the
adduser script, it takes you through creation of a user by prompting for input (such as desired username, required groups etc) instead of having to type in all the options initially.
Hope this helps!