Reptiles are fascinating creatures with species specific requirements, and correct husbandry is paramount to your friend's health and happiness. Schedule a visit with a veterinarian who specializes in exotic medicine soon after you bring your reptile home. During this wellness visit, the veterinarian will do a complete physical and will determine if any infections or health issues are present and will also help educate you about proper care to prevent health problems in the future. THE MAJORITY OF ILLNESSES IN REPTILES ARE CAUSED BY IMPROPER OR INSUFFICIENT HUSBANDRY, so it is extremely important to learn how to properly care for your reptile. An experienced exotic pet veterinarian can help to educate you. Information found on the internet should be backed by veterinarians who have experience with exotics pets.
Despite all the differences in husbandry present in the reptile world, there are a few things they have in common and parameters that need to be monitored daily.

TEMPERATURE: Maintaining the correct temperature is extremely important for the health of reptiles. The habitat should provide a temperature gradient, so that one end of the enclosure is warmer, and the other end is cooler. Reptiles cannot control their own body temperature, so they move from hot areas to cooler areas as needed to control their internal temperature. Each type of reptile has its own Preferred Optimal Temperature Zone (POTZ), a range of temperatures at which that species will grow and thrive. Furnishing your reptile's enclosure with rocks or other species-appropriate objects will allow additional temperature gradients, and will allow your reptile to hide or climb to satisfy his natural instincts. 

Generally, the minimum temperature should be no lower than 75*F and the maximum should not exceed 100*F, but remember this will vary with your specific type of reptile. Heat lights should be used to provide the warmth that reptiles need. The goal is to warm the entire environment, including the air in the enclosure. Heat rocks and under tank heat pads can be dangerous, because they do not supply sufficient heat, and the reptile will get burned from close contact. Red bulbs and ceramic heat emitters are recommended to supply heat without light, to avoid artificial daylight 24 hours a day. 

It is essential that a thermometer is used to measure the temperature inside the habitat. The thermometers that adhere to the side of the tank are not accurate and should not be used. Invest in a good digital thermometer and place it in several different areas to make sure that you are creating a temperature gradient consistent with your reptile's POTZ. 

UVA/UVB LIGHT: Reptiles require both UVA and UVB rays. The UVB rays supply Vitamin D3 that allows reptiles to absorb the calcium in their diet. Without UVB, reptiles cannot absorb calcium and will get very sick. Calcium is required to keep your pet at its healthiest and is necessary for almost every bodily function. Calcium absorption is especially important in young, growing reptiles, and females that are producing eggs. Species who live outside get their UVA/UVB requirements from the sun but for those kept indoors, it is critical to provide a source for UVA/UVB rays. The UVA/UVB bulb may mimic sunlight, but direct sunlight is best for all reptiles. Sun filtered through glass loses the important UVB rays, so it is recommended to take your reptile outdoors in a controlled, secure manner for sun exposure several times a week. For indoor UVA/UVB bulbs, ideally get a long fluorescent style bulb so that the entire tank receives the light. The good wavelengths of light will decrease over time, so it is important to replace this bulb every 6 months. When you are purchasing your bulb, make sure to read the box carefully to ensure that it truly supplies UVB radiation in the 290-320nm range. 

HUMIDITY: Humidity requirements vary greatly between reptile species. Sustaining the correct percentage of humidity within the habitat is vital to the well-being of reptiles and will help prevent a host of different diseases. An accurate hygrometer is the only way to measure humidity levels.

SUBSTRATE: The bottom of your tank should be covered with a species appropriate substrate to help maintain heat and humidity. Many types of substrate are available but generally, reptile carpet is preferred. This carpet can be removed from the tank, washed, disinfected and then reused. Sand in any form should be avoided, as it commonly causes impaction when the reptile ingests it. Bark can be a problem as well, if it is small enough to fit in your reptile's mouth. Sometimes reptiles will accidentally ingest bark or bedding material while they are eating their normal food or prey item, so you may need to feed them in an area without bark or other small bedding types. 

FURNITURE: Reptiles need a place to hide within their enclosure. This will decrease stress if your reptile feels threatened and needs a place to get out of sight. Ensure the hiding place is big enough for the reptile's entire body to fit in. Furniture is needed by some reptiles to help them shed, especially snakes and lizards. There are many options available at most pet stores but something as simple as a coconut shell, thick foliage from a non-toxic plant, or a good sized tree branch can be used. 

WATER: As with all living creatures, reptiles need access to clean water. A good, fresh water source should be available at all times. The way reptiles drink is varied. Some will not drink from a water bowl, and others will submerge themselves completely. Chameleons drink drops of water hanging from leaves. It is important to learn about the particular needs of your reptile.
Education is key to the health of all reptiles, and proper care and husbandry will allow a long and enjoyable relationship with your pet. 

"Reptiles and amphibians are sometimes thought of as primitive, dull and dimwitted. In fact, of course, they can be lethally fast, spectacularly beautiful, surprisingly affectionate and very sophisticated." -David Attenborough