The gunslinger above has it nailed (the others are correct too). Also, don't trust the little pre-filled 510 cart tips in most kits.... many report no-nic effect from them. Get some good, new, e-juice. Nic content 18-24 mg (this is highly variable and subjective, however). You have to find out what works for you. There are some videos by Mistress Nomad in the stickies talking about this stuff.
Other common things: Spare atomizers, some "cut" solution so you can lower the nic (and flavor) if you wish and a drip tip.
Welcome and good luck!