Let’s start from the beginning.

All tanks have a base stat that you can achieve with a 100% crew (the commander is 100%, and the rest of the crew goes up to 110%). If this is the case, then your reload time, camo rating, vision range is the base amount that is specific to the tank and depends on the modules that are put on the tank. (better turret = better viewrange, better engine = faster acceleration/top speed) These base stats can be boosted by fitting extra equipment to the tank (such as gun rammer = faster reload, coated optics = better viewrange, ventillation = +5% on the crew skills). You can also train specific skills after you’ve reached 100% (and 110%). At first, you should go for Sixth sense for the commander, and if you’re not playing a light tank, go for repairs for rest of the crew. The light tanks have to go for sixth sense + camouflage on the rest of the crew. For second skill, go for brothers in arms.