You can control which areas of the file system are indexed, and how files are processed, by setting variables inside the Recoll configuration files.
You can also use multiple indexes defined by separate configurations, typically to separate personal and shared indexes, or to take advantage of the organization of your data to improve search precision.
The first time you start recoll, you will be asked whether or not you would like recoll to build the index. If you want to adjust the configuration before indexing, just click Cancel at this point. That way, recoll will have created a ~/.recoll directory containing empty configuration files.
The configuration is documented inside the installation chapter of this document, or in the recoll.conf(5) man page. The most immediately useful variable you may interested in is probably topdirs, which determines what subtrees get indexed.
The applications needed to index file types other than text, HTML or email (ie: pdf, postscript, ms-word...) are described in the external packages section