First of all, you have to determine your Max Heart Rate (MHR). The absolute best way to get this done is to do what is called a "VO2 Max Test" at a local university - this is the most accurate measure of MHR. If not, the old standard 220-age is a good starting place, it works for about 70% of the population (not for athletes). Another really "fun" test, which I DO NOT recommend for beginners is this:
Wear a heart rate monitor (I recommend Polar brand), go to a 1/4 mile track and.............
1. FULLY warm up for at least 20-30 minutes (longer if you are prone to injuries or over 40, like me), walking, jogging, stretching, etc.
2. Run 800yards (2 laps) as fast as you possibly can - you should be miserable, hating me, etc.
3. Rest 30 seconds (keep walking and ONLY rest 30 seconds)
4. Run another 800 all out. Now you really are hating me. You should be ready to puke at the end of the 2nd lap.
5. The highest heart rate you hit in the 2nd 800 should be very, very close to your MHR.
Once you determine your MHR, make most of your cardio workouts average 80% (70-90%) of that number (i.e., if your MHR is 170, the majority of your workout - not warm up or cool down, those should be lower - should average 136bpm (beats per minute)).
If you don't want to go to all of this trouble, you can use the "RPE" scale (rating of perceived exertion) and keep it around an 8 out of 10, but this is VERY subjective and usually not accurate. You can also use the old standard, "talk test": if you can talk normally while doing cardio, you are not training hard enough. If you cannot talk (unless you are in a short interval) you are working too hard. Your speech should be labored and broken, you can talk but you have to breath between words.