Dogecoin is one of the old coins which has been trading on many exchanges for a long time. You can get started by visiting coinmarketcap, search Dogecoin and check the exchanges it is trading on, but personally I would suggest you trade it on Binance.
Now to trade on Binance, first create an account with your email or phone number as the case may be, confirm the registration and set up Google authenticator (2FA) in order to protect your account from theft. For easy access, download the Binance mobile app, and login thereafter, then goto your fund section, search for Dogecoin, and click on deposit to get the deposit address, then copy it. With the deposit address, head to your wallet and transfer the Dogecoin to the wallet address you copied from Binance.
Once the deposit is successful, this you will know by email, then open your Binance mobile application, go to fund section and you will see the Dogecoin, click on it and select the trading pair you want that is, Doge/BTC and so on, click on the trading pair, fill in the amount you want to sell and place the order, when it executes you will have Bitcoin.
In some other exchanges, it is almost the same process, you just have to follow it slow and steady, and the hang of it.