You have to go to a broker and open an account. And then place an order to buy one share of whatever company, and pay with your credit card. You will be charged a commission, which, depending on the broker, may be a significant portion of $100, i.e., the commission may be $25 or $30. It’s not the wisest way to invest.
If this is for practice then don’t buy one. The commission and fees will be too high. Open a Yahoo account and go to Yahoo Finance. You can set up a pretend account and track your stocks daily at no cost and no risk. You can “buy” some risky items with high potential returns as well as safe items that plug along with low but consistent yields.
@downtide I think it is a great way to get to know the market and the unwritten rules. Try it without actually spending any money. Pretend you are Warren Buffet. “Invest” a million dollars and see what happens. It is free.