Find a deleted file.

Unless you moved a file to the Trash on your Dock, but have not emptied the Trash, then the file is for all intents and purposed gone, and unrecoverable. There are file recover tools on the market that might be able to recover all or part of that file, if you used them immediately after it was deleted. Otherwise, not long afterwards, the operating system reuses that deleted file storage for another write operation, and recovery is impossible then.

I strongly recommend purchasing a large external drive, and commencing the use of Time Machine to backup your Mac.

Sorry to hear that.

Have you looked in your Time Machine backup? Assuming you have been backing up with Time Machine there is a good chance the file is in TM, you just need to locate it at a point in time you knew it was on the computer.