While I can't give direct investment advice, I can walk through how to think about your different options.
But before you choose to place additional risk on your money by investing it, I highly recommend making sure you have appropriate savings first.
The exact number will vary based on your comfortability, but in my opinion, you should have at least one month of your living expenses set aside before investing.
Because if you invest prematurely and don't have other savings in cash, you may be forced to sell your investments at a loss or pay unnecessary taxes to access your funds if you need the money. I call this the financial walk of shame.
If you have to sell investments early, you may miss out on a lot of growth.
For example, based on historical performance, if you miss out on just the FIVE best days of the stock market during your investing career, your returns drop by 35%.
So, when you get started investing, you want to stay invested.
The last thing is to consider your goals.
Is the $100 a starting point for long-term investing, or are you trying to trade stocks in hopes of making a quick profit?
I tend to stay away from trading and instead take a long-term approach because I don't want to think about my investments all the time (and you have a better chance of making money in the long term).
Here's a graphic I made that shows the odds of making/losing money based on how long you're invested:
If you're only invested for a month, there's a good chance you might lose money. When you take a long term approach (5+ years), your odds of making money go up dramatically (89-99.8%).
But let's say you're ready to get invested and put your $100 to work - you first need to choose what type of investment account you're going to use.
Here are a few different options based on what your goals are:
Taxable brokerage account
IRAs (Traditional, Roth, Simple, SEP)
401(k) or Solo 401(k)
Alternative investing platforms
After you choose your account type, then you can choose your investments.
You may decide to invest in the stocks of individual companies you believe in or you may want to take a more passive approach - which means you could go with a few broad-market ETFs (i.e. bundles of different stocks).
Again, I like to take a passive, long-term approach. So for example, a classic set of investments is a 3-Fund Portfolio. The idea is to pick a group of simple, low-cost ETFs or mutual funds that give you plenty of diversification.
Here's an example using Vanguard funds:
You may choose to go 50% S&P 500, 25% international, 25% bonds. Or depending on your risk tolerance, maybe you pick some individual stocks with 25% of your money and stick with a total stock market and international index with the other 75%.
(If you're under age 50 and can mentally handle the ups and downs of the stock market, I wouldn't recommend investing in bonds because they're lower risk and historically provide less returns than the stock market)
Also read: How to build an investment portfolio
The nice thing is that with most modern investing platforms, you don't have to pay full price to buy one whole stock - meaning if Apple's stock was $200 and you only had $100 to invest, you could buy 1/2 of a stock with only your $100.
This is called fractional investing.
And if you put $100 in tomorrow and don't invest more for another year, that's completely okay. Simply getting an account open is a big step and reduces future friction because you're more likely keep investing & transfer extra money in the future if the account's already open.
My last thought: If the $100 could be invested into a skill or tool to help you make more money, it could provide you much more returns than it would in the stock market. You could then take the money you make and have even more to invest.
While there are no guarantees, an investment in yourself can be just as, if not more, valuable than putting money in the market.