When you're looking for information on a specific substance, PubChem should be your first stop. You can access it either directly through the PubChem link, or by selecting the PubChem Compound option from the dropdown menu on PubMed's homepage.
PubChem will provide you with a range of information on each substance, including the structure of the substance, what companies sell it, where people are exposed to it, and what dangers are associated with it. The Literature and Information Sources sections on each substance link out to a variety of articles (scholarly and non-scholarly) with more detailed information. PubChem is an open science database, meaning that the data it contains has been uploaded by a range of scientists, corporate entities, and government agencies.
ChemIDPlus is like a dictionary for chemicals, and it is maintained and updated daily by the National Library of Medicine. It is somewhat more technical than PubChem, but the records it contains may be more complete/current because it is updated regularly. If something in PubChem seems dated or incorrect to you, ChemIDPlus is a good place to double check.
PubChem may sometimes link you to one or more of these.
HSDB is more specific to toxicology than PubChem. It contains information on substances considered by the US government or scientists to be potentially hazardous. It contains information on issues like human exposure, industrial hygiene, emergency handling procedures, and regulatory requirements.
CCRIS contains information specifically about chemicals that are either carcinogenic or inhibit tumor growth. Its data is more technical than either HSDB or PubChem (you may need to draw on some reference sources to understand it if you're not familiar with the subject area).
HazMap provides data about chemicals and agents that impact occupational health. Each entry provides the lethal dose of the chemical and a very brief summary of the adverse health effects associated with it. The entries link out to other government databases with relevant literature and information.
This is the government agency that responds to environmental health hazards, hazardous waste, and resulting health issues in surrounding communities. Because it's partly intended for community members (non-scientists) to use, it generally provides some good plain language summaries of CDC standards for particular hazards and explanations of the health risks associated with them. It also provides information about specific cleanup efforts.
ECHA is similar to HazMap and HSDB, in that it has information on chemical hazards and regulations. It's an interesting comparison to the US sites because the EU generally regulates chemicals more stringently than the US does.