See this support article:
You can also download at least some of your content (audiobooks being a notable exception) again from the iTunes Store:
For additional instructions, particularly for content not purchased from the iTunes Store, check out this user tip from TuringTest:
and this page on "How-to Geek":
Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.