Start with what you have* and actually read it.
The type of resource that you start with -- case, law review article, regulation, statute -- will influence your research plan. Generally, you can formulate a research plan by reading what you already have and using it to:
* if you have nothing, go to a secondary source (law review article, book, treatise, nutshell, ALR) on a subject you are interested in and read it.
Update each primary source electronically using KeyCite or Shepards to make sure your case or statute is still good law.
Evaluate each resource to establish the proper amount of focus you want to give it in your writing.
Stop when you keep retrieving the same relevant resources and when those resources begin to repeat the same citations and quote the same authorities in footnotes, etc. (Warning: make sure you have pulled all of these cited-to works and authorities and either added them to your list of resources or rejected them for some reason.)