Using FreeCad

I am attempting to design a regurgitating liquid reservoir for a project I am working on, which is really not much more than a hollow hockey puck with a couple holes in it, but every time I try to go into part design, and modify the basic cylinder, it tells me I need an active body object, and I can't figure out how to make my drawing active as I have no real experience with any kind of CAD software, and also I am not very smart.