Stock exchange traded funds - ETFs have had a pretty good long term yield over the past few decades without too much risk. For example, an S&P 500 ETF is a good choice. Next question: how to get into one where the fees & overhead are minimal. A company called Vanguard has low fee ETFs and you can open an account directly with them.
A word to the wise: many firms offer "commission free trading." However, there's always a fee; it's often hidden in small differences in the buy price and sell price and it's hard to compare because these prices change second by second. So, starting out you have do your own research and go by reputation. In any case, trading around a lot and switching funds WILL cost you.
Some funds go for stocks that pay dividends every quarter; others go for high growth. You'll have to choose your strategy.
