A very common piece of advise: Do your research, and know what you're getting into before you buy your reptile.

This. I can think of no greater bit of advice

Don't just assume something because someone else said this or did this. Looking up things yourself, getting books or even go to experts. Because it doesn't hurt to find out as much as you can before getting the animal. Along with after that point make sure you have the time to actually take care of whatever it is you get. Because many people don't understand how much time it can take taking care of some of them. For example a full grown iguana.

Do your homework, research about the species you want, don't buy an animal with high or specific needs if you're new to reptiles, and finally, DON'T but your animals from per shops, get them from respected breeders!