Happy early birthday!
What do you vape now? Do you chase flavor or clouds?
If you are a flavor chaser get a LP Reomizer to start. It's the gold standard of Reo atties. If you like a little more cloudy, then the RM4, 5, or 6 would be good choices and come straight from Rob.
A good starting point is a LP (low profile) Grand or Mini as they can take any atty you throw at it. A Grand uses an 18650 battery and holds 6 ml of juice. The mini uses an 18490 I think and holds 3 ml of juice. I don't own a mini so I'm not positive on the battery size.
Also get 1 or 2 spare springs. If you have a short the springs are designed to collapse, saving you and the Reo.
Get some extra bottles so you can easily change flavors. The Reo comes with 2, but with all of Clark's wonder juices 2 just ain't enough.
Appropriate batteries and a charger if you don't have these already. Rob recommends AW IMR 1600 or 2200 for the Grand. These can handle 24 and 20 amps respectively so you can easily venture into 0.4 ohm coils if you wish. These are button top batteries, but you can also use flat top batteries. I personally use Sony VTC4 and 5 and Samsung 25R.
Authentic AW Batteries - RTD Vapor is a great battery supplier.
Enjoy your venture into Reoville! I think you will love it, I love mine!