How could I start buying and selling?

My only sources of income ATM are EMA (�10/week) and my grandparents (�5/week) so at jjust �15/week it would take a long while to save up for something that cost just �150. Any way, I have been wanting to go into buying and selling goods (preferably bikes) for a long time now but have never known where to start (on top of the fact that my Dad has threatened to cancel my debit Card if I buy stuff without his knowledge but I just found out that he was bluffing the whole time) so could someone tell me the basics? All I know is thhat you have to find something to buy, find someone who is willing to pay more than what you paid for it + posting. Then, when you get it just have them wire the money to your account and then send it to them in the post, that's it though, I don't know anything else