PLANNING the DVD, and ACQUIRING the ASSETS (or creating them) are the MOST important first steps of making a DVD. 

What are assets?

Assets are any media that you use in the creation of your DVD - Video, Audio, Graphics, Subtitles, whatever!

Where do I get them?

Well, usually you make them - but sometimes them are provided already digitized. There are many pre-fabbed graphics sites on the web, as well...

OK - How do I create them?

Video Assets can be movies or pre-encoded MPEG-2 assets - In many cases, you will be editing this video, and will only have to encode it for MPEG-2

Menu graphics are usually created in Photoshop, but can also be created as Motion video in Adobe After Effects, or Apple Motion

Audo assets for DVD are usually either PCM audio (AIFF, WAV) or Dolby Digital 2.0 or 5.1. In the case of Dolby, a Professional encoder will be required in order to create properly licensed and compliant Dolby Digital audio streams. As luck would have it, Apple's COMPRESSOR is a Dolby-licensed professional AC3 encoder, so you're ready to go if you have that. 

Other assets (like Subtitles, and Closed Captions) are created in prooprietary desktop programs, or created by special vendors who create Subtitle or Closed Caption files.