I've never used bitcoin, how do I invest real money here/get more back?

I've never used bitcoin, how do I invest real money here/get more back? I notice a market feature. I am not familiar with how these work(aside from basics of buy now-low, sell later-when it's high). I simply want to pull say $20 off my credit card and watch it grow here and withdraw the grown amount when it is higher. I get error messages when I try to simply "buy" I'm very confused. Please give me detailed instructions. I've used computers my whole life and am generally competent, I will understand, nobody is explaining this though.