The first step to running as a limited user is to create a new account on your Mac to take over the administrator role. To create a new account, open the Accounts preference pane by clicking on the Apple menu, choosing System Preferences, and clicking on Accounts.
Underneath Login Options, click the plus sign. If it is grayed out, you will need to click on the lock symbol and enter your username and password to unlock this option. Once you click the plus sign, a popup will appear asking for the new account information. Be sure to select Administrator in the New Account pull down menu.
Give the new account a username, password, and password hint. Afterwards, click Create Account and your new Administrator account will appear in the list of accounts on your Mac.
More information about creating new accounts is available in this Apple support document.