When you get a Crew, the first thing you'll want to do is start earning Crew XP in order to train them to 100% proficiency. The amount of Crew XP you gain after a battle is equal to the tank's earned XP. There is no Crew XP penalty if a Crew member was injured or killed during the battle. Here's the amount of Crew XP needed to train a Crew member to 100%:

To go from 50% to 100%, it takes 48,314 Crew XP

To go from 75% to 100%, it takes 24,157 Crew XP

Once your Crew reaches 100%, you will then be able to start training Skills and Perks. Once the first Skill reaches 100%, you will be able to train your Crew in a second Skill. You can continue to train your Crew in Skills, one at a time, until a maximum of 25 different Skills. Be sure to read the descriptions for each Skill, and select the best Skills for your play style. The Crew XP needed to train each Skill and Perk will double every four skills:

For the first four skills, it takes 100,800 Crew XP per Skill/Perk

Costs continue doubling every four skills, up to a total of 25 Skills/Perks

To help earn more Crew XP per battle, Elite and Premium tanks have accelerated Crew training. Accelerated Crew training provides 25% more Crew XP at the end of every battle. This allows those tanks to train Crews faster to get the Skills and Perks you want.

Premium tanks are the best vehicles to use for training Crews. Unlike Elite tanks, Premium tanks have an XP bonus that earns your Crew anywhere from 10% to 70% more tank XP per battle before the accelerated Crew bonus is added. Since tank XP earns Crew XP, these bonuses are invaluable when training a Crew!