Hello and welcome!
I haven't been doing this very long but I have made my share of mistakes. Here are a few of my thoughts...
#1 An ro/di unit is a must. You can buy it at stores, and many people are just fine with that, but imo just go buy one if you can. I got mine on ebay from user purewaterclub*com and have had great luck with it. The water I make is almost perfect (4ppm or less) and saves loads of time. I got a 6 stage unit for less than 80 bucks. Here is the link to them
http://stores.ebay.com/purewaterclub
#2 Be patient. Everybody says it. Few actually do it. Going fast is almost always bad. Tank maturity is a major role in health and success.
#3 Do your homework. No matter what you are buying, whether it be equipment (tank, lights, adhesives, stand, etc...), livestock (fish, inverts, corals), or anything else for you're tank (live rock, sand, etc...), read up on it. There is a ton of info here and other places on what to do, what not to do, what to buy, what to avoid, etc... The internet is an invaluable tool. The more research you do ahead of time, the less money you will waste. I know if I had known then what I know now, I would have a few extra bucks in my pocket.
#4 Get access to a digital camera and take pics of everything you do. It's so much easier to show people rather than try to explain it and it is fun to look back and see how far you have come.
That's all I have for now. I am sure I will remember more dumb things I have done and post them here. Good luck and have fun.