Hi there and welcome
It appears Mike is not online at the moment, do you want to wait for him or continue with me?
Please let me know
ok, do you want to reset the whole Macbook so it is as new again and you can set it up like just buying it from the shop?
You loose the data fro your cousin on it though but then it is as new
If you wish to return it to new, you can do the following
Power off the mac
Then start the Mac and hold down the Command and R keys at startup.
Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window
There you choose to Erase Disk, then you choose to reinstall the OS which will return it to a new mac so you can set it up again like new.
Let me know how you go please.