Your broker, Ameritrade, offers a variety of Exchange Traded Funds (ETFs) that you can buy and sell with zero commission. An ETF is like a mutual fund, but you buy and sell shares the same way you buy and sell shares of stocks. From your point of view, the relevance of this is that you can buy and sell as many or as few shares as you like, even down to a single share. Note that to get the commission-free trades on the available ETFs you have to sign up for it in your account profile. Be sure to do that before you enter any buy orders.
You'll want to start by looking at the Ameritrade's list of commission-free ETFs. Notice that they are divided into different categories: stocks, bonds, international, and commodities. Which categories you pick from will depend on your personal investing goals, time horizon, risk tolerance, and so on. There are lots of questions and answers on this site that talk about asset allocation. You should read them, as it is the most important decision you will make with your portfolio.
The other thing you want to be aware of is the expense ratio for each fund. These expenses reduce the fund's return (they are included in the calculation of the net asset value of the shares), so lower is definitely better. Personally, I wouldn't even consider paying more than about 0.10% (commonly read "10 basis points" or "10 bp") for a broad-based domestic stock fund. For a sectoral fund you might put up with as much as 20 bp in expenses. Bond funds tend to be a little more expensive, so maybe allow as much as 25 bp, and likewise for international funds. I've never invested in commodity funds, so I'll let someone else opine on appropriate expense ratios for those.
Once you've decided what funds you want (and have signed up for commission-free trades), all you have to do is enter the trade orders. The website where you manage your account has tutorials on how to do that. After that you should be all set.
Good luck with your investing!