It can be a bit tricky task if you are not familiar with magento system, but it’s pretty easy once you know where to go.
Open magento administration page, then go in system->permissions->users.
Click on “Add new user“, fill necessary data.
Be sure to click on the Roles tab (on the left) and associate Administrator Role to user.
Click on Save.
Of course you can also create new role and assign to user some non devastating permission (i.e only ability to change descriptions and photos of your products).