if you are new to DITA and want to create a custom DTD configuration with or without custom specialization this tool is a good starting point.
you may ask why, at least if you do not want to introduce custom semantic to your data model you should at least define which domain belongs to you. a summary why to go this way, read http://drmacros-xml-rants.blogspot.com/2007/04/dita-standard-practice-always-make.html.
Online version of "DITA DTD Generator" is available as online version. source code can be found here: http://code.google.com/p/dita-generator/.
If you have to support custom semantic (which is common in enterprise usage of DITA) you might use "DITA Visual Specialization Manager" or read "DITA Specialization Tutorial".
As always using the right tool is only the baseline. The more advanced task is to identify what you need based on your business case.