One of the most basic and inexpensive tools you can use as a cyclist, after you have bought your bike of course, is to buy a heart rate monitor. They start from very simple models, up to the Garmin 1000, the Rolls Royce of HRM’s (at the time of writing).
For the beginner cyclist, I would recommend going with one of the big brands, Polar or Garmin. Those that come with a chest strap are by far the most accurate. I would also not recommend a wrist-mounted system (watch) as it can be dangerous and distracting to read whilst riding, especially in a group or around traffic.