What do you mean, how do you preview your video before exporting it? Are you asking how do you preview to see what a compression level would be like before you compress the entire footage, or what? I mean, if you're just wanting to see what you've chained together, the canvas window (the view screen on the right above the sequence) plays back what you have in the sequence - so technically that is your preview prior to exporting. If you're wanting to see what a given compression level would look like, that would require you to export a portion of your video and then check how it looks prior to exporting the entire thing...
If you don't have the canvas window (on top there should be three windows - your one containing your assets, the preview window and the canvas window) go to windows->arrange->standard and it should pop up.