Just take it nice and slow. No one starts out at this game knowing it all. Just take it nice and slow. Asking questions is always the best way to start.
First thing you need is an ohm meter.
ALWAYS have an ohm meter. You never want build a coil without an ohm meter. I don't know if you've had a third degree burn, but I have. It took two months for the skin to fully cover the burn (it was from a pizza oven, not a battery), and then another month for it to heal.
Now what you need to know is Ohms Law. Ohms Law is the relation between the voltage of a power source, the resistance of the conductor, amount of amps drawn, and the wattage produced. Why do you need to know this? Because if too many amps are drawn from a battery (this is determined by the battery's C rating and capacity, we'll get to that later) the battery will experience a dead short, enter thermal run-away, and vent hot gasses and/or explode. This is very bad for you, the customer, and business.
This page will help.
As will this one.
Go on YouTube and look up Rip Trippers tutorials on building. He's nut, but he's good at what he does. He'll teach you the basics.
Also, remember when building you want to leave about five amps of head room in the battery. So, if you have a 20A, you don't want to go past a 15 amp draw so you won't want to go past 0.28 ohms or so.
Generally, here's the equations you need to know.
Amps (C; current)=Volts (V) / Ohms (R; Resistance), or, C=V/R
R = V/C
Watts (P; Power) = V^2/R, or, P = V^2/R
Now, generally,
http://www.steam-engine.org/ can help you out with Ohms Law, coil wrapping, and making sure that you aren't going to dead short a battery, but you still could. So use your ohm meter.
Even if you aren't going to be building coils for your self, you need to know this to tell your customer who are going to be asking you this stuff.
EDIT: I just realized I never did get to C ratings. C ratings are complicated, and I'll admit I don't fully understand what they mean. As far as I know the C rating is how many times it's capacity a battery can safely discharge without over heating. They're simple to use though. You simply multiply the C rating (it will be a number, not always a pretty one) by the battery's capacity in amp hours (most capacities are given in milliamp hours [mAh]. To get it in amp hours just move the decimal accordingly; IE, for a Samsung R25, you have 2,600 mAh capacity. To get it in Ah, just adjust the decimal three places to the left. You get 2.6 Ah).
So, for a Samsung 25R, you'll have C of 7.7 and a mAh of 2,600. To get the Maximum Continuous Discharge rating, you'll move the decimal of the capacity three places to the left to get 2.6 Ah, the multiply by the C of 7.7. You'll get about 20.02 amps which rounds to a nice even 20 amps.
The Sony VTC4 has a mAh of 2,100 and a C of about 14.29 so it has a MCD rating of 30.009 amps.
I just want to add that I haven't been vaping nearly as long as some of the other members of this forum. I don't have the depth of their knowledge on the subject. I just like to be informed and have spent far longer than is probably sane researching the topic. I like vaping as a hobby, and it's saved my life. I used to be a two-three pack a day smoker. I want to be sure that what has saved me from a sure death by any one of the diseases brought on by smoking tobacco won't kill me through my own carelessness, and, by extension, share what I've learned to make sure that others will also be safe.
Ask question, learn more.
