Do as much research as you can and decide what it is YOU want out of an e-cig. How big can the e-cig be (battery and e-juice delivery device), how much
vaping time you want, whether you want one you can use removable/rechargeable batteries in or one that needs to be plugged in, (there are also devices you can vape on while it's charging too!) do you want a simple device or one with a little bit more in options like a variable volt or variable volt/variable wattage product (that will make a difference in the "hit" you get from each puff.). If you opt for the ones that look closer to a cigarette, you're gonna make a trade off for vaping time (they are usually smaller batteries), and possibly be limited on what type of e juice you can vape (some only come with pre-filled cartridges whereas some offer blank ones so you can fill them with your juice of choice.) Also decide what your budget is. Remember, everyone is different, thus their needs and desires in an e-cig is different. Think about what yours are and arm yourself with knowledge!
If you go in with some ideas, you'll be less apt to be pressured into buying something that isn't really what you're looking for.
When I was looking around, I had done as much research as I could and still didn't know which e-cig would be the "one" for me. So I pretty much settled on one that had variable volts/variable wattage, was size changeable, used batteries I could charge separately so I could have one less worry about needing/wanting a vape while it was plugged in. And I wanted one that would take the greatest variety of e-juice delivery containers as possible. When I talked to someone in a store, they wanted to dissuade me from my choice and tried to sell me something that would have cost me (just for their kit no juice!) more than the one I was thinking of. In fact, for roughly the price of their kit, I got a great e-cig, some clearomizers, a great charger, great batteries and about enough DIY e-juice and flavors to last approx 8 months! What I settled on (because of it's versatility) was something I felt would satisfy my needs today, tomorrow and for whatever they might evolve to. Also, their kit was not "tailorable", it was limited in comparison to the one I had chosen. Because I had done my research and had a few "must haves" and a few "don't minds" in mind, I was better prepared and knew what wouldn't work for me.
Good luck in shopping and Happy Vaping!