If you use Lightroom, from the menubar select "Lightroom" -> "Edit Watermarks..." that will let you make a simple watermark based on available fonts on your system. It should become obvious when the watermark editor window appears... it'll let you pick typeface, size, placement, and transparency.
If you want to use artwork as a watermark (not just text), then you need to create the art with something like Photoshop. But the trick is that it should have a transparent background. JPEG files do not support the concept of a "transparent" pixel in an image ... but PNG files do (as do TIFF images, but I think Lightroom only accepts PNG and JPEG as watermark files and you don't want to use JPEG). So... just save the art as an image with a transparent background then tell Lightroom to use your image file.
I'll be candid... I really dislike large, obvious watermarks. I prefer not to have them at all -- they 're distracting. And while the watermark doesn't really stop someone from copying your image, if they remove the watermark in the hope that they won't get caught, removal of copyright mark is a DMCA violation (and has a stiff fine. Whereas the simple copyright violation for a non-registered image is not worth much.)
For this reason, I often watermark my own images but I try to absolutely minimize the distraction of the watermark (it's tiny, it's in the corner, and it's mostly transparent. In some of my images you would really have to know where it is and be looking for it to see it.) I also add copyright info into the meta-data of my images for the same reason. If someone goes through the effort to both remove the copyright message from the meta-data AND remove the copyright watermark on the front, it's hard to make the case that removal was accidental.