To search your entire library, make sure My Library is selected in the left hand menu. If you want to search only those references in a particular collection, select that collection.
You can organise your library by using tags (another name for keyword). Authors sometimes assign tags to their published papers and these will be included automatically in the references you have imported into your Zotero library.
You can add tags to your references manually:
To search your library for articles with a particular tag, simply search the
tags box on the bottom left-hand side or browse and select the appropriate tag.