Joomla already offers a built-in RSS/Atom Feed function using a Core-Component, but it's really hard for new users to implement and use. Some facts about the built-in RSS Feed Component:

• You may assign a maximum number of item contents (articles), which will be shown in the Feed.
• You may only assign RSS Feeds to whole Section(s) and/or specific Category(-ies); the articles that are in those Sections and/or Categories will be displayed in the Feeds.
• You may have as many RSS Feeds as you want; every single Section and Category can have its own Feed.

It is wise to use a "read more" link in every article, so that only an intro will be displayed in the Feed.

1. Combine Joomla's Feed with the Power of FeedBurner

The best way to go is to combine the built-in Feed Component of Joomla with the free RSS Feeds service of Google called FeedBurner. From the Site of FeedBurner:

FeedBurner's services allow publishers who already have a feed to improve their understanding of and relationship with their audience. Once you have a working feed, run it through FeedBurner and realize a whole new set of benefits.