It's hard to lose weight on a bike if that is all you are going to do. The better approach is to monitor calories in and out making sure that what goes in is less than what goes out.
To lose weight using a bike, the longer you ride the better, but if you are new to cycling than you don't want to go so long, at first, that it puts you out of commission for days afterwards, either with sore muscles (leg, back, or neck), sore joints, sore glutes, or saddle sores. Go easy at first, until you build fitness. After a while, you can start to ramp up the intensity of your rides, up to lactate threshold (LT)--the point where your leg muscles feel like they are 'rigging up" (as in your muscles feel like rigor mortis is setting in) or you feel a burning sensation in the muscles. If you've reached this point, you are working too hard. Back off until this condition clears.