I've never done this in anything except Adobe Lightroom. It's a nice piece of software but probably more than you want to get into, unless you are a serious photographer. So check into the above suggestions.
When you post an item online, the pixel count should be low enough that no one else can use it for a print. That means re-sizing it to about 72 pixels per inch which is good for web viewing but not printing. You can do this old Microsoft Office Picture Manager, which can still be downloaded. There is probably other free software to do it, too.
A determined person can remove a watermark, but the above precaution will make it non-profitable.
A decent watermark will prevent the casual thief (and that's what it is) from lifting your images.
Any time you take a picture, you automatically own the copyright, regardless of watermark. You can request that any site using it remove it, and they usually will. If you need to, you can get a court order to make them. However, if you want to seek monetary damages, you must register the image with the US copyright office. You can google how to do that, it is not difficult to do in large batches.
Depending on the camera you have, it is possible to embed copyright info in the image itself, so it can be seen with a right-click. I don't know of any way that can be removed, but a lot of people don't know about it, so it's a good way to prove an image is yours.
Single empty-nester in Middle TN, sometimes with a friend or grandchild on board
