Open system preferences (found in the apple menu)

In system preferences there's a black icon at the bottom it's a picture of 2 people and says 'Accounts'. Open that and you'll be halfway there. Make sure the little padlock icon in the bottom left corner is unlocked. (If it's locked just click it and put in your login password to open it.) Just above that are 2 buttons a '+' and a '-' Click '+' That will add a new user. The input fields to the right let you give the user a name and a password. You can also impose restrictions on their access to your system under the "Limitations" tab. I believe the default position is No Limits.

Hope that's helpful...

Jeff