If you see an outstanding Bug or Feature Request that you want to fix, great! However, be careful about just diving in and starting to write code...
Firstly, check the Owner field to see if someone's already assigned - there's no point in duplicating effort!
Next, what is the Status of the ticket? If the Status is Open, then more work is required before coding should start.
For Feature Requests, it means a decision still needs to be made as to whether or not we want to have this added to the project. Join the discussion and add your thoughts to the comments on the ticket (you can also use the voting buttons).
For Bugs, we need to verify if it actually is a problem and document the steps required to reproduce it. As this could require a certain amount of work in it's own right, you might want to assign it to yourself while carrying out the investigation. Don't feel that just because you've done some investigation that you have to see it all the way through to fixing the issue - you can document what you've found in the Comments of the ticket then unassign yourself, someone else will pick it up at some point.
Once a ticket has the status of Verified, then by all means, start coding! Assign the ticket to yourself, checkout the latest version from the SVN trunk and make your changes. When you're at a suitable point, add your changes to the ticket in the form of a patch file. It doesn't need to be the final "finished" thing - we don't mind applying patches incrementally to a development branch, in fact it's not a bad idea since it allows others to review progress.