trying to work my way in...confusing though

I'm now about to start my second online work at oDesk as a Data Virtual Assistant. It looks like a cool task, but I'm a bit worried on how to use the oDesk tool. I haven't tried it once, and I hope I get to use it correctly. I'm a bit nervous as things run my mind...what will I do if the tool doesn't work th way it should? How would I be billed with the work I'm doing.. I can't put it on hold much longer as I might lose my client..I really hope all works well..