Undeclared earnings in 10/11 - concern over registering for SA now

A few years ago I started making websites as a hobby. In July 2010, I decided to start monetising these assets in order to covering my costs. Before that tax year was over, I'd made over £1.5k (I was at college with no other income). I didn't declare this income (it didn't even cross my mind: I was concentrating on getting into university!)

In the 2011/12 tax year profits increased ten fold and I now subsequently have to complete a SA return. However, to register I have to answer the question: "When did you start working for yourself?". I'm very concerned that if I state July 2010, I will be massively fined for not completing a SA return for 10/11. Will this be the case and if so what should I do next? I want to pay tax but I don't want to get into legal problems at the same time!

I'm a UK citizen and have never dealt with tax before.

Many thanks for your time.