To buy Dogecoin, you'll need to open an account on a cryptocurrency exchange. This exchange is an online place where buyers and sellers meet to buy-and-sell cryptocurrencies. Some popular exchanges include Coinbase, Binance, and Kraken.
If you don't want to go through the trouble of opening an account, you can also consider using the investment platforms that you already have. For example, some brokerages like Robinhood allow you to buy cryptocurrency directly from their platform.