I like to use btc-e, the site has an easy to use and intuitive front end as well as it allows you to trade in automatic mode. As far as cashing out you have an option to transfer the funds to a few popular online payments systems.
So you're aware the value of bitcoins is probably more volatile than the stock markets? And that unlike the stock market it's not guaranteed to have an overall upward trend? You'd probably have an easier time putting your money elsewhere ...