You need to create an Action first - this is where you record the steps you go through to watermark your images.
Actions are in the History palette.
You will have to make your watermark in a separate file, it has to be the correct size to go on your photos, make it on a separate layer from the background, delete the background layer and save it as a .psd with a single layer with a transparent background and close the file.
Open an image you want to watermark.
Create a New Action
-In the Actions palette, create a New Action.
-Click the New Action button, name the Action, click the Record button.
-File > Place, choose your watermark file, position the watermark with the Move tool.
-Click the Stop button at the bottom of the Actions palette.
You may have to practice this first before you record your action, I always stuff it up and have to do it a few times.
After you have made your action, you go File > Automate > Batch
Choose the Action that you have created, set the parameters for saving it and you are away.
HTH