A few initial tips:
1. When initially starting an international law research project search for research guides in that area of international law that can provide useful background information, context and starting points. One way to locate such guides is simply Google: International ____ Law Research Guide and see what you find. See, e.g., International Food Law Research Guide, International and Foreign Animal Law Research Guide, etc.
2. Resist the temptation to go directly to a treaty relevant to your area to read it and attempt to interpret it. As with most legal research go to secondary sources first. What books are available on this issue or this topic? What law reviews have looked at this issue? And if you have an issue that has any historical component, make sure you also look for law review articles on Hein, which usually go back to volume 1 of most journals - remember that Lexis and Westlaw generally do not. Check Google Scholar and other non-legal databases as well.
3. When you think you have exhausted what's available in secondary sources, go back and check again, make sure there are not more out there that you can use to see relevant analysis and to identify relevant international instruments, relevant case law, other authorities, etc.
4. When you finally start researching primary materials such as treaties or UN Resolutions, find a research guide about doing that kind of research. With treaties, always check what entity is the depositary (which should be outlined in the treaty itself) for that treaty and make sure you consult the official list of who is, and is not, a party and what reservations, understanding, or declarations there may be. It is often easy to find the text of a treaty somewhere online, but very few of those online postings of treaties can be trusted to do the work of continually updating the status of parties.
CUNY GUIDES
CUNY Law, Quick Guide to International Legal Sources
CUNY Law, International Law Research
TUTORIAL
Duke/Berkeley, International Legal Research Tutorial
RESEARCH GUIDES
GlobalLex (NYU), International Law Research
CUNY Law, International & Foreign
TREATIES
ASIL Research Guide, Treaties
United Nations Treaty Collection
HeinOnline, Treaties and Ageements Library
U.S. Department of State, Treaties in Force
TRAVAUX PREPARATOIRES
Yale Law School, Collected Travaux Préparatoires
Jonathan Pratter, À la Recherche des Travaux Préparatoires: An Approach to Researching the Drafting History of International Agreements
CUSTOMARY INTERNATIONAL LAW / STATE PRACTICE / GENERAL PRINCIPLES
NYU, Researching Customary International Law, State Practice
UNITED NATIONS
United Nations, Research Guide
