Well, is there something that you don't like about 0ad? Is there a feature that would amaze you? Pick one, check with some people, file a ticket, write code, submit a patch.
More precisely Well, picking one, go wild.Ask on #0ad-dev if they would accept that feature (stay around, you don't get immediate answers!). Or open a forum thread, but I guess that is even slower.If they like it, open a ticket on trac, to keep track of development and provide a place to comment.Write code, don't be afraid to ask, people are nice on #0ad-dev. (at least towards me )Submit a patch with your feature to trac (your ticket). Prepare for criticism.Code some more, to appease the critics, then go back to the previous point until the patch is accepted.Or even better: If you can figure out how to fix a bug, then it doesn't require any creativity and you aren't likely to be rejected, as with certain features.
Keep in mind that I am not part of anything official. This is just what I am currently doing.
Edit: feuneur was refering to the simple tasks found over there: http://trac.wildfiregames.com/query?status=assigned&status=new&status=reopened&keywords=~simple&col=id&col=summary&col=status&col=type&col=priority&col=milestone&col=component&report=16&order=priority