Windows 8.1 and Ubuntu Gnome help

So, I have a problem. I have a step father that has been going on to my computer, for no reason. So I was wondering if I could do someone on my two hard drives, both Ubuntu Gnome and Windows 8.1. Encrypt them and make it so you can't use something simple to crack the password. Free is the best thing to do. I know some of you guys are going to say things like "If you have nothing to hide, then don't worry" or "He is just worrying about you", something along those lines. If you are going to say that, please do not comment, I hate him doing that, I would not do that to his computer, so why should he do it to mine. He has full access to my computer when I am at school. Which is about 6/7 hours. I am planning on using a laptop that I have with Ubuntu Gnome to set up a small security system that would record when I am at school and maybe when I am asleep.