Summarize this article:

Even though they're left to ferment for a long time, the apples you choose can significantly shape the flavor of your finished vinegar. Choose the best quality apples available to you in order to get the best apple cider vinegar at the end.  For a more complex and deep vinegar at the end, try using a combination of different apples. Use two sweet apples, such as Golden Delicious or Gala, with one sharp tasting apple, such as McIntosh or Liberty, for a slightly sharper vinegar at the end.  Instead of using whole apples, save scraps from apples used in other dishes to make your apple cider vinegar. One whole apple is roughly equivalent to the scraps of two apples. Keep the peel, core and other scraps in your freezer until you’re ready to use them to make vinegar. It's always a good idea to wash your fruit and vegetables before eating them, and the same is true when cooking or fermenting them. Give your apples a thorough rinse and scrub with cold water to clean away anything you don't want in your vinegar.  You can use any number of apples you want to make apple cider vinegar. The more you use, the more vinegar you’ll get! If you’re just starting out making your own apple cider vinegar, try using 3 apples for your first batch. This will give you a good amount of vinegar but won’t put too much at risk if something goes wrong.  If you're using apple scraps, make sure to wash the whole apples before separating the scraps from the rest of the apples.

Summary:
Choose quality apples. Wash your apples in cold water.