You can find a free version of this tool on the Jing website. It's made by a company called TechSmith. It's a very simple and easy software tool to use. It does run constantly in your PC background. A small yellow semi circle sits at the top middle of your monitor (you can always turn it off). Once you click on it, you get 3 buttons - Capture, History and More.
I just click on the first yellow circle with the plus sign for Capture. That brings up some yellow guidelines. Position them on your screen in the upper left corner of what you want to capture. Click. Then position down at the bottom right corner. Click. You'll get a small option box where you select 'Capture Image', then another option box to copy or save your image. It's so easy!!
Here's a screen image I cut from the Jing website. You can also write some text on your screen image. Here I wrote "Cute Frog!!".
There's a free version for both Windows and Mac. TechSmith makes other fee-based screen capture tools which probably offer lots more customization if you want to become a serious tutorial writer, but for a free tool, it's a pretty good one!