I can give you one tip- with $500 there are a lot of online trading services that will charge you unless you are activly trading (1 trade every quarter or something). A lot of them (like e-trade (who I'm with) require (IIRC) you to have at least $5000 for that fee to be waived. MAKE SURE you read through the agreement of each.
You might just be better off buying stock through a broker and getting the certificate. When I got my first stock (when I was VERY young), I bought 10 shares of wrigleys at $20 (post split- really 5 shares at 40)- only $200. What is nice about that company is that they have a dividend reinvestment plan (DRiP)- and every quarter I get my statement and there is a little form I can fill out and send in a check to buy more stock directly - skipping the broker. I would suggest going that route with the money you are going to invest (don't necissarily have to get wrigleys (WWY) there are other companies that have DRiP plans)
ETA- There are also many stocks with Direct Purchase Plans (DPP) where you can purchase stock without already having been an investor like DRiP. Here is a link of a whole bunch of DPP/DRiP companies
www.wall-street.com/directlist.htmlMany of theese companies will tell you how you can directly invest in their company under the 'investor relations' portion of the website. For instance here is Procter & Gambles investor relations site
www.pg.com/investors/sectionmain.jhtml and just click 'How to Invest in P&G' and it makes it quite easy to do so.