What are the legal procedures to start an online business for advertising?

I am starting a online business that offers many free stuff for users, and later some people will want to pay by PayPal to advertise on my website. I would like to ask you what are the legal procedures to let people advertise and pay a certain amount of money to me every month they want my online services in order for them to advertise on my website and do all of this legally?

What should I do to make this legal? What are the basic procedures? Please, help me, I don’t know anything about Law and I need to get more information in order to make my online business possible.
Thank you.

I’m in Alberta, Canada.

And also, is it possible that maybe PayPal, Google Checkout and Yahoo! Wallet may help me with this issue?

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