Creating a Watermark

I'm new within the forums AND with Apple and its products. Can you tell me how to add a watermark to my photos?

I will also need to remove this watermark at a later point in time.

Welcome to the forum.

It really helps if you can add someinfo about which Mac you have and which version of OSX. It gives us a bit of a starting place for giving you more helpful responses.
There are numerous tools and applications for image editing, including watermarking. It's hard to advise fully without having a bit of background in what your doing and why you want a watermark etc.

But I'd suggest something like Photoshop Elements is a good investment if you're doing a lot of photo work. But your Mac comes with iPhoto bundled. There's a free plugin from here:
BorderFX - Home that can add watermarks.

The purpose of watermarking is to stop the image being re-used or tampered with. So removal is not an option. When watermarking I'd suggest you create a duplicate, watermark that for distribution etc and keep the original safe for yourself.