Welcome to ECF and wahoooooo on wanting to get off cigs!! You'll get a million different answers as to what is the best thing.
If you want something that looks like a cig, most folks recommend the volt.
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If you want a battery that will last all day and then some on one charge, there are two basic types. (Many more, but I definitely recommend a 510 connection). Something like a kgo, a kit with 2 batts, charger, atomizer etc, will run about $40-50 This is where the battery is contained in the unit. If you want something where you simply replace rechargeable batteries inside a shell, something like the Epower or an Ego T would be good.
The threading means what type of connection it is where you screw on the juice holder. 510 is probably most common. You do need to know which threading you have and the thread on whatever juice holders you're buying. If you get the wrong one, it won't screw on without an adapter.
It can be very complicated starting out and very confusing, but here are my main things I learned from being a recent newbie myself.
Watch videos of different products! Just type in (name of product) review into your browser or into you tube. You'll get to actually see products being used and find out what people like and don't like about them.
When starting to buy juice, get a variety of nic strengths. Folks told me if I was having cravings, up the nic strength, so I placed a large order for 24mg nic juice and then found out that higher nic can give juice a peppery flavor. Now I have a lot of juice that I have a hard time vaping. Also try some various pg/vg combos to see what you like best. Pg can be harsher but gives more flavor and throat hit, vg gives more vapor. EC blends is a good place to try 3 ml bottles to try various flavors, pg/vg and nic strengths. It'll cost more per ml, but overall could save you money by finding out what you like.
Plan to spend some money on this til you find out what you like. I'd also recommend getting a dripping atomizer. I didn't do this with my kit and really regretted it. Many kits come with one. This allows you to drip a few drops of juice directly onto a coil (sort of) and that way you can taste juice flavors. I got the drippers dream atty from gotvapes.com and really like it. Many people just stick with dripping and don't want anything else. But you can only drip about 3 drops, and vape that maybe 5-6 draws (sometimes more). Understand that you will likely end up buying things you don't like, it's a learning process, but so much better than cigs.
Have backups for everything as you can get them. Batts fail, attys burn up, nothing lasts forever. But just remember that with cigs you were literally setting $50.00 bills on fire each time you bought a carton probably. And you've come to a great place! Plenty of support and help here. Good luck!!