Hey, as probably many of you i spend some money on my photo gear and I love it

I was recently thinking maybe I could make some money on it as well? I did few bits and pieces for local school but and would like to start advertising. It all would be great if it would work out however I need to know how to make it legal. I am thinking of getting a website and do some leaflets in my area. I am not too sure if I will make some money of it at all and anyway I would only do it in my free time after work. Is it legal? can I advertise beforehand and them register business? What if I register and it does not work out? What I don't want to happen is to advertise, don't make any money and them the tax man saying you do stuff and don't declare it. Please someone help. I am only thinking of doing something occasionally.