Hi Roxanne. Easiest way to keep track of cadence is a bike computer that has it. I personally use a Polar S725X w/ the power kit on one bike and the cadence kit on the other. But this is probably way more than you want to spend as a beginning cyclist. But the Polar also comes with a heart rate monitor which is useful if you do training based on heart rate.

There are several relatively low cost options for tracking cadence with a bike computer. Sigma Sport BC1600 is one option for instance. Doing a quick google search, this web site (http://www.arequip.com/Categories/Bike%20Computers.htm) has this computer for $27.95. It also lists several other computers with cadence capability.

One thing you want to make sure of while looking at computers is to make sure that the computer you get not only has cadence capabilities but also has it included. Some, such as the Polar, has it as an option. It's not part of the regular kit. So in order to get cadence you will end up paying more than the base cost.

Another option if you're not looking at purchasing a bike computer is to have a clock with you and count the number of turns in 15 seconds then multiply this by 4 to get your RPMs. While riding on the road, I don't like this method since it takes your concentration off of riding which could be dangerous. On a trainer this is fine.

Mel