I would take that 75 bucks and open a savings account just for future investing and add to it weekly or monthly until you hit 500 bucks.
Then I would buy a 500 dollar CD and borrow against it and invest the proceeds into an account stock acct like at scottrade and pay the monty payments to pay off the loan against your CD.
At the end of the payoff of the CD you'll still have you 500 dollars in a CD earning interest and 500 dollars plus or minus in your stock acct.
This is the way I invest, because it makes me save and at the same time gives me the funds to invest.
You can keep borrowing against the CD and invest the proceeds into stocks and grow your investment portfoilo.