Your e-cigarette will have 2 or 3 basic components:
1) the battery - what matters here is how long it will last between charges. That's indicated by "mAh."
2) the atomizer - the battery heats up the atomizer which vaporizes the liquid
3) the cartridge - the part of the e-cig that holds the liquid
Cartridges are disappearing fast. Combination atomizer/cartridges are called cartomizers. There are also other ways to hold the liquid such as
juice boxes (you put a bottle of liquid inside), automatic feeders/
tanks, etc. Or you can just drip a little liquid into the atomizer.
If I were starting today, I'd buy an eGo or a Riva. They're both 510s so it's easy to get atomizers, cartomizers, drip tips, etc. Compared to the basic 510, which is 180mAh, the eGo is 650mAh, the Riva is 750, the eGo megas are 900-1,000. Depending on how much you vape, you can go a single day on one charge. (I vape a lot. I usually have to change batteries unless it's the one for my big Chuck.)
I have a Riva that I bought from
Liberty Flights that I'm very happy with. (I linked to the page with the code for their 20% discount.) It's lightweight, inconspicuous (for stealth vaping) and produces a nice amount of vapor. It doesn't compare to my Chuck -- if you want to start directly with a mod, I think it's the best.
Don't overbuy liquid. Try 3 or 4 flavors that sound good to you. You'll only go through a few ml a day so you'll have plenty of time to reorder and try something new next time.
You can absolutely change the liquids you use in an atomizer. If they are flavors that taste good together, you can just make the switch. For the last few days I've been vaping TastyFinger (think ButterFinger candy bars), Cappuccino and a combination of Butterscotch & Vanilla all on the same atomizer. If you don't think the flavors will mix well, you can clean out your atomizer when you switch by blowing it out or using one of the many cleaning methods you'll find on this site. There are a few flavors that tend to cling to an atomizer no matter how well you clean them. You may want to give them their own atomizers. (Cinnamon, peppermint and some tobacco flavors are like that.)