Just got my first shotgun - a used Browning Citori 12ga. What should I check before taking it out?

Great choice for a first gun - Citoris are tanks! First, make sure it's unloaded (check both chambers). Then, give the barrels a good look down the bores for any pitting or obstructions. Check that the action locks up tight - no wiggle when it's closed. I'd also run a cleaning rod through it and maybe replace the factory chokes if they're old. What's the barrel length on yours? That'll help with ammo recommendations.