Open a Bugzilla Account
If you do not already have one, you should open an account on bugs.kde.org. This will enable you to log in and comment on bugs.
Join the Mailing List and IRC Channel
You should join the bugsquad mailing list to keep up to date on the latest bugsquad happenings. We also gather in #kde-bugs on irc.freenode.net where you are welcome to ask any questions you have. We are happy to guide you through your first few bug triages!
Pick a Bug to Triage
Choose a bug to triage. Pick any bug you like - a good place to start is with an application you use regularly and are familiar with. You can use bugzilla's query interface to search for bugs to triage. Look at newly filed bugs, this is a good place to start. You can modify the search to narrow it to the application of your choice. If a bug has lots of comments on it from different people, it is likely that the developers are paying attention to it. So look for things that have slipped through the cracks.
Find out what needs to be changed
Read through The Steps of Triaging a Bug to learn what to do once you have chosen a bug to triage.
Get your changes verified
Making mistakes when triaging bugs waste developers' time, and they don't like it. So, before making any changes to a bug, you should get a second opinion. You can either ask on our irc channel, #kde-bugs (on irc.freenode.net), or failing that, use the Outstanding Bug Changes page to record the bug and another triager will check that they agree with your proposed changes.
Make your changes
Once your proposed changes have been verified (and likely amended) by another triager, you can make the changes in Bugzilla. If you do not have the required permissions to make a change, you should list the bug on the Outstanding Bug Changes page. Then someone who does have the required permissions will make the change for you.
Repeat steps 3-6 above
Now you have triaged your first bug, repeat the process for as many more as you like. If you triage loads of bugs, you might even find youself mentioned in the commit-digest!