get a good battery, meter, mod and rda. learn Ohms law. Learn the calculations. as a beginner definitely stay away from Hybrid mods. Mechs are easy to use, and are only as dangerous as the uninformed. I have seen newbs trying to fire a .1 coil on a 10 amp battery, and I have seen people who were scared to fire anything lower than a 1.5 coil on a 30 amp battery... Just learn what your doing, and ask questions.
As to your question about when you know your battery is going flat... well basically when the vape isn't hitting as hard, not as much throat hit, not as big a cloud, the HISS from the coils sizzling the juice diminishes... there are a bunch of things that give it away that you will pick up on as you use it. granted you can always stick a multi meter on teh battery to see exactly where its at, but after using the mod for a while you will start picking up on what it is telling you as far as when to swap batteries.