Questions about tanning and what causes it and its prevention and alot more!!!?

1- What causes tanning?
2- How to prevent it?
3- How long would it normally take for me to return into my real skin tone/color?
4- Can this process be fastened?
5- Does sunblock prevent tanning?

The place where my university lies is really very sunny so i am so tanned and my color is really changed and i care about it, so i want to get to my real skin color and not getting tanned again.