Buying shares is typically done through a brokerage account. You can find a list of possible trading platforms here. Many will allow you to buy ‘fractional shares,' which allows you to own portions of stock without buying an entire share. For example, some stock, like Berkshire Hathaway, can cost thousands of dollars to own just one share. However, if you only want to invest a fraction of that, brokerages will allow you to do so.

If you're looking to bet against a company, the process is more complex. You'll need access to an options trading platform, or a broker who will allow you to ‘go short' a share of stock by lending you the shares to sell. The process of shorting a stock can be found at this resource. Otherwise, if your broker allows you to trade options, you can either buy a put option, or sell a call option at a strike price above where shares are currently trading – either way it allows you to profit off of the share price decline.