Instructions for downloading and installing osm2pgsql on different architectures.
The manual contains all the details about running osm2pgsql.
The man page has a short overview about running the osm2pgsql command.
The FAQ is a list of frequently asked questions and answers.
The osm2pgsql page on the OSM wiki also contains some more information, some of it outdated.
See a list of releases.
If you want to set up a whole rendering toolchain to create maps from OSM data, you need more than just osm2pgsql. Have a look at switch2osm to get started.
You'll need to understand how PostgreSQL works to make the most out of osm2pgsql. Understanding at least a bit of the SQL query language and something about PostgreSQL configuration is important.
The PostGIS plugin extends the PostgreSQL database with geometry data types such as `POINT` or `POLYGON`, the geometry operations that manipulate those data types, and special geometry indexes.