Can someone explain this crew thing to me? What do I do with them, and what do they do for me? If i want to sell a tank what do I do with the crew? And when purchasing a new tank should I also take a new crew or use an old one.
You can get larger guides, but some basics:
Your crew first learns basic skills - that means your loader loads faster, your driver drives faster, your gunner aims faster - up to 100%.
After learning 100%, they start to get optional bonus skills. They won't do the basic things faster, but they get new benefits.
The way it works though is that some skills activate immediately and get better as you get exp - for example, repair or mentor give 10% of the skill at 10%, and so on.
Other skills only function at 100% - like "sixth sense" or "deadeye", the skills for your commander and gunner than say if you're spotted, or add a 3% chance to damage other tanks' modules or crews (for example, you might break their gun so it won't shoot until repaired, and then partly work).
Each crew member has a list of skills you can pick.
The commander has a special benefit that he adds 10% (when he's 100% trained) to other crew member basic skills.
One skill that is different: "Brothers in Arms" only takes effect if it's 100%, and only if all crew members have it. It adds 5% to all the basic skills - which translates to about 2% actual increase (for example, your gunner gets 5% more basic skill, which makes him load shells about 2% more quickly, so a 20 second reload becomes 19.6 seconds).
Each type of tanks tens to benefit more from different skills.
Arty probably benefits the least of all the tanks from bonus skills - I find brothers in arms the most useful. After that, clutch braking is nice for rotating, camo I think is overrated because by the time the enemy is close enough for it to help, you are almost always going to get spotted anyway. Repair won't help that often because arty is rarely just damaged long enough for it to help. Sixth sense is handy, but same as camo by the time it goes off, you're in trouble anyway.
You can just add bonus skills as you get them, and you can reset them - but resetting them loses exp unless you pay gold.
The way wargaming set it up, you are tempted to get the skills that have partial function while you level, and then reset and get a skill that takes 100% - making them gold.
For example, say you start your commander with 'mentor' so while you're leveling him, you're getting bonus exp.
But when the skill is 100%, you might want to reset to get 'sixth sense', and add mentor as a second skill for bonus exp leveling the second skill.
That's the basics - it then takes understanding which skills for which tank.
For example, TD's love: sixth sense, clutch breaking, deadeye, repair and camo (off the top of my head). And brothers in arms, like all tanks.
As far as transferring crews:
Crews are one thing in the game you can't buy skill past the basic skill, like you can use free exp to buy a tank or modules. That's a reason to start crews at 100%, spending gold.
You very much want to use the same crew as you get new tanks (once they have enough skill to want to keep the xp; has to be the same country). So you level your crew skills on a tier 6 tank, and when you get a tier 7 tank, you transfer the old crew to it so it starts off with skills and keeps adding skills. Otherwise, when you get that tier 9 tank, you'd start with a crew that has no bonus skills. But here also, except premium tanks, it's set up for you to lose exp when you transfer the crew unless you spend gold for each crew member. For premium tanks, the crew can use any tank of the same nation and type as if it were trained (I spent a lot of gold training crews for premium tanks before I found that out). And premium tanks are used for training crews because of that.
For example, my T110E5 crew is also used in my T34 premium. They get a 2x bonus in each tank daily. My German TD crew has two premiums plus a normal tank.