Since magit 2.1: magit-log-buffer-file (as per comment below)
Before magit 2.1: magit-file-log is what you are looking for. It will show you all commits for the file in the current buffer in the standard magit log view.
Open your magit-status buffer, by typing M-x magit-status (I used to have this bound to C-. C-g because it is used all the time. These days, I use Spacemacs so it's <SPC> g s)
Type l to get log viewing option
Type -- to set the "Limit to files" option (used to be =f)
Enter the file path you wish to view history for
Type l to view the log of the current branch
If you're using Spacemacs, you can get the history of the currently visited file using <SPC> g f h