If you haven't emptied the Trash, you can pull it back out of the Trash. If you have emptied it, it's doubtful you can retrieve it. Search the forum for "file recovery" or "data recovery" and you'll find many threads with recommendations of apps that might help, such as:
Just buy an external hard drive at least as large, preferably larger than your internal drive (if you don't already have one) and watch this video on Apple's site. That should at least get you started.