You can open a money market account at your bank and put the money in it. You can buy a $400 certificate of deposit (CD) from the bank every month, and the bank can set this up to happen automatically.
If you want to invest into a mutual fund, you can set up an automatic investment plan. If you are set up for automatic investments, many funds will waive their minimum investment.
Since you have a 40 year horizon, you should invest in something like stocks that have a higher return but have higher fluctuation. Right now, if the market drops 10% this year but goes up 20% next year, you are fine, since you won't be taking the money out after one year. As you get closer to retirement, slowly move the money into investments that don't return as much on average but don't fluctuate as much. There are mutual funds called target date funds (40 years from now is your target date), and they will start you out in higher earning investments with more fluctuation and will slowly move you toward investments with lower return but lower fluctuation over time.