Whether or not you can recover a deleted file depends on several things. If you did not empty the trash and the file was not on a server then simply double-click on the trash icon and drag it out of the trash. If the file resided on a server it was immediately trashed and you'll have to contact the system administrator about recovering the file. If you emptied the trash there are programs that can sometimes recover a deleted file but their success is highly dependent on how quickly you realize the file shouldn't have been deleted and whether you continued to use the computer after that event.
In addition, provided that you have turned on Time Machine for backups, you should have a local backup of the file, and entering Time Machine from the menu icon on the top right of your screen (it looks like a backwards-facing clock) should let you find and recover it.