Never, ever, impulse shop, especially when buying fish. If you see a fish you like, put it on reserve, most LFS will do this, and then come back the next week. If it's still kicking, it should still be kicking when you get it home. Or know exactly what species you want before you buy.
Always have a quaranteen(sp)/hospital tank and several medicines ready to go!! It doesn't have to be big, it doesn't have to be pretty, but you'll never regret having one. Don't be tempted to fill it, the best thing is if you never have to use it, and it's just a pretty vessel of water. The best quaranteen/hospital tank is an empty one.
Learn the scientific names for your fish.
In my opinion, my top tip is patience. Never rush into anything in this hobby. Never rush into cycling, never rush setting up, never rush to stock, never rush to breed. Take your time and be patient and you won't regret it.