1) I dont know how to write a watermark, like a copyright notice, on a photo.
2) I have a folder of photos which I wish to watermark with the same watermark.
How should I write a watermark on a folder of photographs?
This can be done in Lightroom very easily and in Photoshop with a few more steps but it is overkill to spend money on an expensive software just for watermarking. Some people even do it from the command line using a shell command and free nconvert utility.
One extremely important thing to pay attention to is to watermark on export. Most people do not watermark their original files, that would not be reversible in case you need one day to make a print or a different watermark.
For this, I think I'd use ImageMagick. It's free and oriented primarily toward batch operations like this. The ImageMagick web site has samples for doing watermarking.
Edit: As @Sean pointed out, it is worth noting that ImageMagick is a command-line based tool, so to use it you need to create the right command line. It may not be the most suitable tool for beginners, especially those who've never used command-line based tools.
1) This is simply a matter of adding your watermark on a separate layer above your photo and any adjustment layers. It can be text or a logo - anything you like. Then simply lower the opacity so the watermark isn't too prominent. You can also change the Blending Mode of the Layer to something like Overlay to help it blend in a little.
2) The simplest way is to record step 1 as an action, then use File > Batch to apply the action to a collection of images. Bear in mind that this doesn't give you any control of the placement of the watermark in every image: if you recorded the action to place the watermark in the centre of the image, it will be in the centre of every image.
You can automate this using a Photoshop Action.
Basically you want to add a text layer, then set the layer opacity down to level that looks good (test on light and dark images). You can add layer style "bevel and emboss".
This looks to be a pretty good tuturial on the subject:
If you want several files watermarked at once, or a folder full of files, you can use my watermarking program SquiggleMark. (Disclaimer: I am the author of SquiggleMark)
It is free to use.
You can position your watermark, resize it, set its transparency, and watermark a bunch of files all at once. Works on Windows, Mac and Ubuntu.
I am personally doing it like this: http://www.paintshoppro.com/en/pages/watermark-photos/ it might be simpler. You should give it a try. ;)