You should create a user account for each person who regularly uses your computer. Each user has a separate home folder and can adjust his or her settings without affecting other users.
To create a user account:
Choose Apple menu > System Preferences and click Users & Groups (Accounts in <=10.6).
Click the lock icon to unlock it, and type an administrator name and password.
Click the Add (+) button below the list of accounts.
Choose a type of account from the New Account pop-up menu.
Administrator: An administrator can create, delete, and modify accounts, install software, and change system settings.
Standard: Standard users can install software for their own use and change settings related to their accounts, but can’t administer other accounts.
Managed with Parental Controls: For these accounts, the administrator can restrict access to applications and inappropriate content, and limit the amount of time users can use the computer.
Sharing Only: Sharing-only users can access shared files, but can’t log in or change settings on the computer.
Group: A group account contains other user accounts and is used for setting privileges for shared files.
Enter a name for the account.
A short name is generated automatically. If you want to use a different short name, enter it now. (After the account is created, you can’t change the short name.)
Enter the account password in the Password and Verify fields, and then enter a hint to help the user remember the password.
If you want the home folder encrypted for security, select the “Turn on FileVault protection” checkbox.
Click Create Account.
If you want the user to be able to have administrator privileges, select “Allow user to administer this computer.”