It's really easy to do on Adobe Photoshop...
So, you have your picture open. Be sure your layer window is also open cause it will make things easier (go to Window and be sure that "Layer" is checked).
Select the insert text tool from the main toolbar (it's a "T") and select the area where you would like to insert the text Once you've selected the area, go ahead and type whatever it is you would like to type, and change the font color to a color that contrasts your image (white is always a good option..you can also set it to your desired color before you even begin typing which is easier).
Once you've got your white text on the picture and you are happy with it, select that layer of text (the text is layered over your picture) on the layer toolbar and right click. From there a little menu will pop up...chose "blending options".
Then a new thing will pop up where you can edit how the text is blended with the picture. Under "General Blending" you'll see a thing where you can change the opacity of the text. Set it to about 30% (or whatever other value you fancy). This will make it so you can see through 70% of the white text which is essentially a watermark. If you want to make it look fanct, like adding a bevel or some kind of shadow, there are also options for that in this same toolbar on the lefthand side.
I don't know how good those instructions were. It takes less than a minute to waterpark a picture using that procedure once you figure out what you're doing. Here's a finished product of a watermark using that procedure:
I'm sure there's an actual watermark tool on photoshop or something that's even easier, but this method works fine for me.