For any number of reasons - security the highest among them - it's a good idea to reset the account password on your Mac on a regular basis. Assuming you remember your current password, here's the simplest way to do so.
1) Make sure you are logged in to your Mac with the account whose password you want to change.
2) Open System Preferences under the Apple menu.
3) Choose the Accounts pane and then click on your account name. It should be the top one in the list.
4) On the right side of the window, click on the Change Password button.
5) Enter the old password (to verify that you have the right to change the password), then enter your new password twice. Your new password should have at least 8 characters and be a combination of upper and lower case letters, numbers and keyboard characters. It's not a good idea to use a family member's name, a pet's name or some combination of your name.
6) Enter a password hint. (This step will help you out later when you forget your password.)
7) Click on the blue Change Password button and you're done.
8) Quit System Preferences.
Changing your password in System Preferences keeps your keychain in sync with your account. You will continue to have access to any other saved passwords (email accounts, wireless networks, websites) if you change your password using these steps.