When you are ready to explore the patent literature
What you already know and your motivation for searching the patent literature will guide your work:
By number. With a patent number or patent application number, finding the document should be not too hard. It should be just a matter of finding a database with the right content and matching the required format. Espacenetis a good choice. Doing a regular Google search with the title or patent number can also be successful.
Keyword searching is appropriate for exploratory work that can be less than comprehensive or as a strategic approach to classification searching. Google Patents is a good place to do that.