I agree with Kalidurga that you should have your local bike shop spend time to do a bike fit with you. This is a process that can take a little time so it's usually best to call ahead of time and make an appointment. Depending on the extent of the fit session there can be a charge for the service so be sure to ask about that. If this isn't an option for you then just Google for "bike fit" and get a friend with more cycling experience than you to help check some of the adjustments.

It takes training to increase your pedal cadence. You won't be able to be a 70 rpm rider on Monday and a 90 rpm rider on Tuesday! You have to develop your muscles and nervous system and aerobic energy system to sustain higher cadences. Best way to do this is by interval training. For cadence work I find it is easiest to do the training on a stationary bike (I also train with a fixed gear bike, but most people don't use on of those). You can find a number of cadence interval drills on line.

Perhaps more important than cadence speed is the form of your pedal stroke. You can pedal very quickly if point your toe down but you'll lose power in your stroke. When you pedal, focus on pulling back instead of pushing down. In other words, at the bottom of your pedal stroke think about trying to scrape gum or mud off the bottom of your shoe.

So get that bike fit checked out, focus on good pedaling form, use interval training to increase your cadence rpm, and most of all, just enjoy riding your bike!

-traveller