My mac is getting spied/display mirrored help!!

I was recenty watching a documentary with my father on BBC, after it ended he said that he had a really good documentry on his computer about the subject that he wanted to show me, lets cut the cackle he was switching to the HDMI port on the TV that he plugged his computer into, when he was switching i saw my desktop, my dock (layout) and my favorite browser game ''ikariam'' opened safari -my city all that stuff-.He somehow was able to mirror my screen, or actually was able to use my computer and reach my data Beside, for a few days facebook was always asking me to re-login -which you know- never happens when you check the ''remember me'' box

please guys help me, tell me a way to block this, i am willing to do anything, take out my hard drive browse its contets at linux, seriously.. anything. I spend almost all my day using my computer. Im able to write this because my dad is assleep because its 4:30 at where i live