Ok,Im 15 And i have just started a small computer business while i finish of my courses.Once you set out what you need to do (Register business name,ABN,Possibly Insurance,GST,Tax info,etc,etc) its pretty easy,just make a ist,do each thing on that list,and after you have completed that list you are good to go.
Next would be to figure what you are going to charge,i charge by the hour (I have done research on other local computer businesses,and i made sure im on of the cheapest,You dont want to soud TOO Cheap) plus parts.I Give the Customer the option of me fixing there computer at there house,or they drop it off at my workshop (Really my house,but workshop sounds more 'professional').
Now you would have to figure out advertising,i have a website (www.lukespcsolutions.info
) and business cards,also flyers and of course,word of mouth.Once you have all that sorted you just have to wait for your customers.I Only started out about 1 month ago and i have had a few customers.
That's Basically what i did to start my business.