You don't have to join the The Ubuntu QA Teams to test prerelease versions of Ubuntu. (These prerelease versions are also called spins, ISOs, builds, or drops) However, as each prerelease version of Ubuntu is released by the development team, it results in a flurry of required testing.
Each individual tester is responsible for downloading a build. Create an account and Start by clicking the most recent milestone under "Testing Tracker" on the QA TRACKER. Find the image to download and click the icon to the left.
Read about filing bugs on our wiki or the bug squad kb.
It's difficult to file bugs that are useful to the software developers. It's nearly impossible for newbies to write a useful and readable bug report. Sadly, developers have no desire to investigate poorly written bug reports. They know that most likely, it will be a waste of their time, as the bug is difficult to reproduce, reported against the wrong version of their software, or simply a mistaken report. Ask for help on the mailing list with writing bug reports.
You're not finished. Even if your bug is very serious, Ubuntu may not deal with the bug before release, if you don't report it on the qa tracker. If the bug persists into the next milestone release (beta, RC1, RC2, etc), you will need to re-add the bug.
The first time I used the qa tracker, I had no idea what to do. First off, on the list of ISOs, instead of clicking the icon to the left, you will be clicking the name of the ISO to submit your result. Read this and Select the testcase to submit the bug against. Read this to submit your result.
You are done. If you have a useful and detailed bug filed in Launchpad, and the bug marked on the newest milestone release in qa tracker, your bug has a good chance of being dealt with in the next build of Ubuntu.