I've been using CircleCI for a while and I can tell you that it is one of the best CI solutions: it is free for personal use, giving you unlimited repositories and users, you have 1,000 builds minutes per month, one container, and one concurrent job; if you need more, you can upgrade the plan with an initial price of $50 per month.
The first thing you need to do is Sign Up on the site using your GitHub account (or Bitbucket, if you prefer). If you choose to use GitHub, you need to authorize CircleCI in your account, as shown in the following screenshot:
