Affiliate Marketing Secrets

I am going to run a blog. I am confuse that should I purchase a web hosting plan or should start blogging on blogger.com or wordpress.com ? Because I heard that if I will make my blog on blogger.com or wordpress.com then they will take the share from my earnings by that blog. Is it true?
But some are suggesting me that I should make a blog on blogger.com or wordpress.com because these are free and purchasing a web hosting plan is a bit expensive.
Guide me what should I do?