How Do I Get This Out Of My Head…?

the way i think has kinda changed. its like i always feel like bad stuff is gonna happen for some reason.. like i get “what if ” thoughts. im 15 and i hate thinking this way. so many wierd things has been happening to me eversince october. i was in global history one time and we had a drugs and alchoal and stuff and im used to hearing this stuff because ive been hearing this since forever. but i startded thinking i just starded high school “what if” i start drinking and smoking or “what if” i have to go rehab. i also remember when i heard about steve jobs death i heard he had cancer and i thought to myself wow cancer can happen to anybody and then i starded thinking oh “what if” i get cancer or “what if” i die or something.i get scared very fast and easily and come up with these thoughts.. i hate it and i never thought this way before……im worried that my thinking will become much worse and i will get stressed easily and im only 15…how do i stop these thoughts from happening next time and how do i be a non stressful person? my dad is kinda like this and he gets worried over stupid stuff and i really dont want to end up like him.. how do i stop this before it gets worse and start being normal and not worry over every little thing? thank you