Most reptiles require special (UVB) lighting and heat and need to be UNDER that special light and heat a certain number of hours per day.  You need to know what your specific species requires.  
 

Some reptiles require certain humidity, either low or high.  Know your reptile.
 

The substrate (material at the bottom of the enclosure) may accidentally be ingested by your reptile.  Depending on the type of reptile, it might be best to feed outside of the tank.
 

Some reptiles live a VERY long time.  Know what you’re signing up for.
 

Some foods are more healthy than others for your reptile, whether your reptile eats vegetables or insects.  
 

Some reptiles are vegetarians, some insectivores, some carnivores, and some are omnivores.  It is important to know the most nutritious foods for your type of reptile, including which vegetables or insects are more nutritious.