Ok, where to start. Well First off, after only a little over a year at this hobby, i'm by NO means a pro yet. But I have made my fair share of mistakes and learned some lessons.
First big mistake i made... I filled my tank with Tap water. I did treat it with stuff from a local store to remove heavy metals etc, but it was still tap water.
I think i did ok with the ammount of sand i bought, i went with 90lbs in a 90 gal. It gave me about 2-3" sand bed BUT i didn't rinse it nearly thoroughly enough and the water would become SOOOO cloudy every time there was any movement in the tank. It took many filter pads to filter that fine silt out of the tank over the next few months.
Next. i cut corners and bought a really cheesy skimmer. Too embarrassed to admit which. After the hair algae got out of control i tried upgrading to another Cheesy skimmer because it was cheap. It wasn't until my 3rd try that i finally spend some searious money on a skimmer thats been working like a dream...
I also went with a standard FW tank, with no built in overflows. So i had to experiment with god who knows how many different PVC arrangements before i found one that worked.
Then i used an old crappy light fixture that i got with the tank that had a bulb in it that was probably WAY too old.
Im sure there are more, but i've managed to recover from all my mistakes..
Nothing but RO/DI water. Proper lighting, Proper Filtration, and a very happy and rapidly growing reef.