Hello,
Humidity is very important. However, that has more to do with what species you're keeping. For some reptiles having a very humid environment is very important whereas with others it may not be as important. You simply have to do your research and find out what your species requires. Monitoring humidity is as simple as using a an inexpensive analog humidity gauge such as the ones sold by ZooMed and Fluker. However you'll get more accurate readings from the digital guages sold at places like Radio Shack. The digital gauges usually have a temperature gauge as well, often with a remote probe which is a nice feature. As for keeping humidity up or down, that will depend on the construction of the enclosure and the humidity requirements for the species. Don't underestimate the importance of proper humidity.
Matt Campbell