Recommended time using excersise bike for starter?

We've just purchased an exercise bike and it'll be here Monday. The last time I actively rode any bike was back when I was probably 10 or 11, I'm 23 now.

I'm already cutting down on crappy foods etc. and trying to get in as much walking as possible, but want to use the exercise bike to burn more calories.

So I'm pretty unfit, my stamina is rock bottom. So what would be a good starter workout to try? Just go until I'm tired then aim to beat it each time? Go for 20 mins then add time each day?

Also, tips on drinking water, should I drink before, during or after im done working out? Or doesn't it really matter?