That's pretty easy. This tech article from Apple is aimed at Yosemite (10.10) users, but works for all current OSes.
https://support.apple.com/kb/PH18891?locale=en_US
"Choose Apple menu > System Preferences, then click Users & Groups.
Click the lock icon to unlock it, then enter an administrator name and password.
Click Add below the list of users.
Click the New Account pop-up menu, then choose a type of user." Read support article for types of user accounts.