Just open System Preferences and go to Accounts > Login Options. Click the "Automatic Login:" drop-down box and select "Disabled." Please note that you will need to authenticate as an administrator to change any of these settings.
You can further customize the "login window" by choosing either "List of users" or "Name and password." If "list of users" is selected, you will be prompted to click your user account name and give your password in order to login. If name and password is selected, you will need to type in both your username and password to login.