The final drive ratio of your bike is determined by taking the rear sprocket tooth count and dividing that number by the front sprocket tooth count.  For example if you have a 45 tooth rear sprocket and a 15 tooth front you would take 45/15=3.0  In this example your final drive ratio is 3.0.  The higher the number the more bottom end you'll have, but there is a poing where you will also start to lose your top end.  Here is a chart to show you all the final drive ratio's for the most common motorcycle gearing.