There are packages for major platforms. You can install PGroonga easily.
There are separated documents for these platforms. And there is a document for building PGroonga from source.
Here are supported PostgreSQL versions:
If your PostgreSQL is older than them, you need to upgrade your PostgreSQL before you install PGroonga.
See a document for your platform in the followings:
32bit + PostgreSQL 9.5.13-1
64bit + PostgreSQL 9.5.13-1
32bit + PostgreSQL 9.6.9-1
64bit + PostgreSQL 9.6.9-1
32bit + PostgreSQL 10.4-1
64bit + PostgreSQL 10.4-1