I don't know a lot about cor, but I would suggest you focus on being able to contribute as support (buffs, /whm or /sch so you can help with debuff removal) while you build up gear to eventually contribute as a DD.
Gear wise, get eminent gear and 109 your AF and relic asap, use the highest ilevel weapons and ammo you can afford (accuracy is important, and the skill bonus on ilevel weapons is very important). Try and idle in ilevel gear for the defensive properties. Get the roll +5 ring from the AH or the +3 ring from sparks if you need something to hold you over.
Basically do what you can to make sure you're not being carried as dead weight. The rest is making sure you're mostly running with people who are willing to do the same. Definitely know where you can contribute and use that to talk your way into a party. Maybe you can't DD without the right gear, but you'd be effective in a mage party giving refresh, macc or fastcast rolls and helping with support, you just need to be able to do it, and convince people you're able to do it.
Also don't be afraid to bead some wins while you work on assembling a party and gear to be able to down things more efficiently.