1.Invest in an index fund which tracks the broad market. Through boom years and recessions, mutual index funds that track the S&P 500 index have returned about 10% a year. That is a decent return on your money, and there are many non-index funds that fail to make that much for their clients. ^[2] There are many other indexes you can choose to track that may produce even greater gains. Once you choose an index fund and make your first contribution, you can add as much money as you want as often as is convenient without any additional costs or commissions. You are also free to stop investing whenever you want. Dealing directly with a mutual fund company in this way, you don't have to pay any broker's commissions.

The minimum investment varies, but with an Individual Retirement Arrangement (IRA) (USA only) you can open an account with as little as $250. (See How to Open a Roth IRA Account.)
If you have $500, more options will be available to you, and you can be a little pickier. In any event, look for an index fund with a low expense ratio (under 1% is best).