Affiliate Marketing Secrets

I want to open an scottrade account, though I want to know when do I pay for opening stock, and like how does it work? when I buy stock do I pay?

Hi, I’m 19 and I have about 400 dollars that I would like to invest. I’ve looked around online and I’ve found sites such as Etrade and Scottrade, both of which seem like good options for me. However, I’m not very well versed in this stuff and I’ve never dealt with stocks before, so I was wondering if there is a website you’d recommend that is best for beginners? Or one that is cheaper per trade (I think Etrade is $10.00 per trade and Scottrade is $7.00, though I could be wrong on how that works)? I think it’s pretty obvious that I don’t know much about this stuff, so any guidance would be greatly appreciated.
Thank you