Simple Tutorial on Trajectory?

I am having trouble making a missle gradually fall down.

Can someone direct me to some simple tuts on that? Or at least tell me what I am looking for. I’ve found some on gravity, but they are usually about things fall straight down. I found some good things on decay, but nothing that really helped.

I want to shoot something from a cannon and have it eventually start to fall. The few I have found were unnecessarily too difficult.