You can use this chart to figure out 'where you stand' but I wouldn't look too much into it. As to where would you start, you would start where everyone does, in cat5, but I would do what others have told you (group rides) before diving headfirst into that. The whole purpose of training is to improve, whether it is your skills or endurance or power output. How much improvement you will see will depend on how smart you train. For best improvements, you will need a structured plan. For that you can get a coach, buy some books, or ask help from others.