If this is your first bike, it is best to get it fitted for you. Go to a bike shop and they will be very helpful in getting you fitted properly. Do not buy a bike from the big box stores (ie: Walmart, Target, Kmart). They will help you determine what type of bike will fit your riding style and needs. This will not cost any extra and you will put in contact with some great people that love to ride also.
Get fitted.....It will make it that much more enjoyable.