On the your Trello homepage that you see after logging in, you should be seeing your teams organizaed alphabetically, and boards alphabetically within those teams. At this time, it's not possible to change that.
What you can do, though, is star boards to bring them up to the very top "Starred" section that sits above your "Personal Boards" section. It's also possible to drag and reorder boards in your Starred Boards section:
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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