These are questions you should really know the answer to if you're going to offer web services. Though people get what they pay for on contest and crowd sourcing sites. So no surprise there…
I would hardly call crowd sourcing websites freelancing. It gives a bad time to those who actually take the time to setup a business using contracts paying taxes and above all else working directly with a client. Throwing crap into the wind and hoping it sticks is not freelancing.
Confirming once again the typical level of skill service providers on crowd sourcing and contest websites have.
If you have specific questions feel free to ask them. However, what you're asking is a little like asking how to become a medical or legal professional. You should pick up some books or online courses to learn. No single post on a forum is going instruct you how to properly make a website or provide an acceptable service level for clients.