you need to record an "action" so you record yourself doing the process to one image and then you tell photoshop to do the exact same thing to a whole bunch of images:

I've just noticed that there is already an action in photoshop to do this: it's called "Sepia Toning (Layer)". Look in the "actions" tab it's the same one as the history. Open the littlke folder called default actions and find the action. Then hit the little play button at the bottom of the pane. You should get a few prompts for you to edit settings.

You should see something like the pic below. Is that what you are looking for?

You then go to file>>automate>>batch... select your options such as folders etc to work on and then hit the button to run it. You should see your pics coming up one by one and getting changed.

Ps. you must remember to hit the stop recording button once you're done with all of the things you want to happen or you could end upo with some messy results.

PPS. Run the batch on a COPY of your original files, until you know what you're doing, cause it's not undoable really