1.) Research and plan out the tank before buying a single piece of rock or equipment, because this can all change based on what you want to keep in your saltwater aquarium.
2.) Slow down and then slow down again. Dont add fish/coral all at once, allow the time to complete a fishless cycle (you wont regret it, much better way to cycle), do not buy products that help reduce nutrients, etc. until you can reduce and fix these problems basically only using water changes (this will help learn how to properly maintain params without outside help)
3.) NO IMPULSE BUYS! These can crash tanks! That fish you had to have but dont have a qt setup can give your whole system ich, that coral could be something you will not like because it grows to quickly (xenia, kenya tree, blue snowflake coral, etc), inverts may not all be reef safe or allow for other inverts to be present (CC Star, Coral Banded)
and 4 cause its important
Become an active member of your local reef club! (if you dont have one, im sure your closest city/state that does have one will welcome you with open arms) My local group has always been there to help and answer with any questions, good price corals/fish/equipment, group buys, and great friends! And its nice because in case of an emergency you will have access to local people that can help in a pinch! Have fun and stay wet!