RES is a community-made browser extension for Reddit. It adds lots of features, including an inline image viewer, user tagging and keyboard navigation. You can download it for free here. ^(~~this comment was sponsored by RES~~)
The best way is to download and install Lightshot (or any other program for taking screenshots). After it's installed, whenever you want to take a screenshot, press PrntScrn button on your keyboard, select area which you want to capture and press button in the bottom left corner (or press CTRL + D) to upload it. You will get a link to the screenshot, so you can post it on reddit, etc.
Hope that helps.
PS: You can do it without using any program, but it's much easier and better to use one.