Try to wright about something that has happened to you or with friends or family if you can get into detail about it, you can add things in like if you were getting chased by something in a mask you can add more fear and suspense by saying...

'I was running back home through the dense tree filled park on a chilling to the bone night, breathing heavily and I could feel my heart beating out of my chest I did not won't to look back but I was to curious for my own good and saw what what hunt my nightmares, I could only see a mask and an out lined shape of there body giving me no clue to the person was but I could them clutching something in there hand and just that alone scared me to the bone. '

Having details really help when writing a story, also if it is to do with something that has happened to you it will make to easier to express the emotions you were feeling at that time.