HELP please, my house is haunted but i don't know how?

My step dad told me when I came in from school today that he heard something being put down on my desk in my room but I was at school at the time. So he went to check if I had came home early and he opened my door and nothing was there. So he went back to what he was doing. Then he heard something moving about downstairs, he thought my brother had came wondering in but still no one was there. So he went back upstairs, and the same thing happened again downstairs. I was at school, my mum was at work and all my pets were outside. My dad died in November last year and ever since then, strange things have been happening and we have had nothing but bad luck. My mum got breast cancer a couple weeks ago but they caught it early but the day she told me about that, I found out my nana died on the same day as well. I cannot stop thinking about this, no one has died in this house, all my family members that have died, have died in their own homes, so how could any get to my house? We all think its my dad, checking up on me to look out for me, it is really freaking me out and making me uncomfortable in my own home, I just want to literally run out, but if it is my dad, he will always follow me everywhere I go and no matter what I do, i'll be constantly living with my dad and I won't ever feel safe or comfortable. I'm only 15 so you could imagine how i would react to this sort of stuff but i'm absolutely terrified and every time I think about it, my heart pounds and I nearly start to cry, any sudden noise just sets me off. I really don't know what it is, but there definitely is something in this house and it is scaring the crap out of me!