As a 15-year-old with $100 looking to start investing, the best way to begin is by learning the basics of investing and setting up a custodial account. You can start by reading books, watching videos, and taking online courses to learn about stock market basics and how to invest. The most common type of account for minors is a custodial account, which is managed by an adult until the minor reaches the age of 18 or 21, depending on the state. With a custodial account, you can invest in stocks, mutual funds, and other securities, and the money in the account can grow tax-free until it's withdrawn.
Starting small is also important when investing as a teenager. Investing small amounts of money allows you to get comfortable with the markets without risking too much. You can consider investing in individual stocks or mutual funds, but it's recommended to start with low-cost index funds that track the overall market. Investing in an S&P 500 index fund, for example, will give you exposure to the 500 largest companies in the U.S. and can provide a solid foundation for your investment portfolio. Additionally, it's important to remember that investing is a long-term game and that the power of time and compound interest are on your side. If you start investing early and consistently, you can build wealth over time and reach your financial goals.