The first - and possibly most important - step is to select the appropriate database for your topic. There are many different kinds of databases out there, but you're looking for an bibliographic-type resource, that covers the literature of a field. There are many to choose from across all subjects. Here are the most useful ones for chemistry:
SciFinder (Chemical Abstracts) is a comprehensive tool covering the chemical literature back to 1907, and is used primarily by researchers and advanced students. Retrieval on simple searches may be overwhelming, since CAS indexes thousands of publications and millions of patents in over 50 languages. Caveat: You have to register for a personal account in SciFinder and log in before searching.
Web of Science is a large database that covers thousands of journals across all scientific disciplines. It indexes less foreign language material than SciFinder, so it's a good tool for students new to literature searching. But it lacks features for searching chemical information such as structures, formulas, and chemical names.
Analytical Abstracts is useful for searching across about 50 key analytical science journals since 1980. It's not nearly as comprehensive as SciFinder or Web of Science, but you can define an analyte and matrix, and it's easy to use.
Google Scholaris a popular free search engine. It indexes the full text of millions of articles in electronic scholarly journals across many disciplines. While the search interface is simple, there's not much you can do with it other than enter some words and then browse voluminous results. It's fine if you just want to find a handful of relevant articles quickly. But you shouldn't rely on Google Scholar for comprehensive searching, especially in chemistry.