I'm going to be honest and admit that I've made lots of mistakes along the way. Firstly, don't try to make more than one website at a time, or you will end up with two incomplete websites that your unhappy with. Don't pick a topic because of the money you could make on it. Pick a topic you really love, otherwise you will slowly but inevitably give up on it. Another thing I learned is that if you want something done right, do it yourself. Getting someone else to write articles or edit your images is your choice, but its you might not be as pleased with the results. Also, try to keep a secure list of passwords and user names that you use to maintain your website, this will greatly increase your efficiency (I can't count the amount of times I've forgotten a password to an affiliate site or a forum). Lastly, don't violate your hosts or advertising affiliates terms of service. You will get caught, and you will regret it.
I should also mention that Joomla sometimes has issues with IE6, but who cares, everyone should be using Firefox!