What operating system? This should include specific minor version numbers, like "Mac OSX v10.10.5"---not just "Mac".
What kind(s) of source footage? When telling about your source footage, tell us about the codecs, not just the container types. For example, "H.264 in a .mov container," not just "QuickTime".
If you are getting any error message(s), what is the full text of the error message(s)?
What were you doing when the problem occurred?
Has this ever worked before?
What other software are you running?
Do you have any third-party effects or codecs installed?
Tell us about your computer hardware. Be especially certain to tell us about your OS, CPU, RAM, GPU (make, model, driver version), any third-party I/O hardware (e.g., AJA, Blackmagic) and the type of media drives you are using.