Awesome! Glad you want to help, and this is an easy way to get started.
The account you’ll need is a Fedora Account System account (often called a “FAS account”) — and if you’ve signed up to post here, you already have it. Almost everything in Fedora uses this single account.
Each such day will link to a specific wiki page, like this one for a CoreOS test day which explains in more detail the specific event and links to resources you’ll need.
You’ll need to be officially added to a Fedora team before you can edit the wiki (we needed to do this to prevent spam), but editing it won’t be necessary to participate — just show up. As you get more comfortable and involved, someone will sponsor you into the QA group. (See docs on officially joining the QA team.)
Most test days use the Freenode IRC channel #fedora-test-day for primary communications. Sometimes there are other channels too — the one above mentions #fedora-coreos. (If you are unfamilar with IRC, using Element/Matrix to join is probably the easiest way. I’m leaving this as a placeholder, because we are in the midst of setting up an official Fedora Matrix server and everything is going to change at that point. I’ll fill in better instructions as we have them.)
That particular chat channel may be empty most of the time but will have helpful people during the test day itself. Feel free to ask anything; “there are no dumb questions, just things we haven’t explained clearly enough” applies.
When you’ve completed a test called for by that particular test day, the https://testdays.fedoraproject.org/ app is also where you log your results — use your FAS username and the test cases from the wiki.