Starting my business aged 21 meant I didn't have much business experience, so I just asked lots of questions. It's a brilliant way to get ideas and solutions to problems.
Biggest career lesson. The value of your network, and the need to invest in it. I used to hate networking but it's a lot harder to grow a successful business without it. You do have to make a conscious effort to reach out to people, but it pays back in the end.
Best piece of advice. Always remember the 'sunshine test' - ('would you be happy for your friends/peers to know what you are doing?'). I find it useful when making any type of business decision.