Just like pretty much every other type of legal research and practice, the best first step is often a secondary source. A good secondary source will pull together all the relevant administrative code sections and related materials. In the case of administrative law, this secondary source will often be:
See the Secondary Sources section for information on how to find these resources.
Here are some additional things to keep in mind:
1. Don't forget that every administrative action is grounded in a statute. Always identify the authorizing statute. It must be provided with the regulation. Or, start by researching statutes.
2. Examine the Code (for federal this is the CFR) as a whole. Do not read just one section. Remember that it is a code and organized such that the sections are meant to be read and used together.
3. Pull up Code sections in Lexis or Westlaw to locating citing references. Use this to find helpful secondary sources, relevant cases, and agency decisions. This will not always work with state administrative codes, but can be very useful for federal.
You might also want to spend some time on specific agency websites. Often they provide a vast amount of information.
To locate state registers and codes, you might be able to Google [state] legal research guide. This works with mixed results. Georgetown provides basic resources for all states.
The Westlaw and Lexis databases above contain both federal and state agency materials. However, they are not comprehensive.
The best place to start is often the agency website. Or get crafty with a Google search, using inurl:gov or filetype:pdf to narrow your results.
FOIA: For every federal agency, there must be a link regarding the Freedom of Information Act. This is almost always at the bottom of the site, and may show up as Freedom of Information Act or FOIA. Clicking on this link should lead you to the FOIA Library, which often provides especially helpful documents, along with frequently requested documents. This is often the fastest way to locate agency manuals, guidelines, memos, etc. --documents that don't show up in the federal register. For example, here is the OSHA FOIA Library.