Prevent Data Loss

I will give my laptop to a third party for repair purpose, and really do not want them to see any of the data.
So how to prevent them? Password-Lock account? Any proper tips.
Of course, I assume it won’t be 100% but at least please advice on maximum methods.
Answer #1
Actually, There are a number of ways to make your data 100% inaccessible! But I cannot get into details without knowing more about the state of your laptop first. What exactly is wrong with it?
Also, Is it your only computer or do you have more than one?
Answer #2
If it is hardware related issues with the machine (other than HDD), remove the drive.
If it is software related, the repair person will have to have admin access to “fix” stuff.
Remove what is private and put it back once the machine is stable. Or better yet; keep all sensitive stuff on external media.