Is the password to unlock the keychain the same as the password you use to log in to the computer? If not, you need to change your keychain password to your computer password. To do so, open the Keychain Access program. It's in Applications -> Utilities -> Keychain Access.
Click on the "login" keychain and then go to the Edit menu. Select "Change Password for keychain 'login'". It will ask you for your old password and your new one. For the old password, use the current keychain password. The new password should be your current login password.
If that doesn't fix it, or if the passwords were already the same, try performing a keychain first aid. To do so, click on the login keychain, then go to the Keychain Access menu and select "Keychain First Aid". Type in your password where it asks for it and then click Repair. Then log out and log back in.
If that still doesn't fix it, trying performing a permission repair. To do so, go to Applications -> Utilities -> Disk Utility, click on Macintosh HD on the left sidebar, and then click "Repair Disk Permissions".