Know what you need to buy before you buy it. Don't always trust your LFS
to know the answers to everything, they are business people, and some will try to sell you things you don't need.
Respect your livestock. They are living creatures just like us, and should be able to live out and enjoy their lives. Don't buy damsels too cycle your tank so they can live in ammonia soup and be killed just because they are cheap and "expedable", there are plenty of other methods.
Don't buy on impulse. if you don't know anything about a species, put it on hold, go home and research all you can about it, and make your decision later. NEVER trust someone if they say it only eats algea!
Have Fun. If you do your homework and know what you're doing, this hobby can be the most fun and rewarding hobby you can come across