Chameleons are rainforest species, and as such require warm temperatures, which is best created through the use of basking bulbs or ceramic heaters. There is a huge variety of options within this, based on the enclosure you're using. With mesh terrariums, a clamp lamp with Mercury Vapour Bulb providing both heat and UVB is often the best option. However, in enclosed wooden vivariums, a heat source with separate strip UVB bulb is recommended instead.
The main thing to remember, however you chose to heat your Chameleons environment, is that there needs to be a basking spot with heat coming directly above, and a the bottom of the enclosure should be cooler, by around 15C (specifically dependant on the species). Any heat source should be controlled by a thermostat to ensure its kept at the correct levels, and you and your pet will always be safe.