I am a couple months into this and already made a few bad purchases.
Number 1 advice: patience.
Research research research and then ask the forum.
You don't want to buy a skimmer and then find out it has design flaws.
Also buy for more than you need. IE higher GPH pump and choke it back as you will find you can split it to go to a skimmer, phos reactor, calcium reactor if your tank demands one.
Make sure the inside of your stand is expandable to house more equipment.