You join crews by applying for them - either in specific crew application topics, or posting a topic within the 'LFC & Recruitment' forum. You should show a range of artwork, both large pieces and tags/signatures, so you truly show your abilities/potential. Battling in the 'Battle Arena' is simple - post a request for a battle ('Battle Requests'), choose the terms of the battle (type of sig/how many people can enter/wager, if any/etc.), and then wait for a reply that you accept. Battling in the 'Battledome' is slightly more complicated - its much like the battling terms in the Battle Arena, but you must pay Draven a 100 credit enrollment fee before you can begin battling there. You also have to wager credits when battling in the Battledome.