Read a bit about rpm (e.g., man rpm). Then download rpms for your distribution (e.g., from ftp://ftp.postgresql.org/pub/binary/v7.0.3/RPMS/). The simplest way to install them would be:
rpm -ihv postgresql*
but you should really get familiar with rpm and basic Unix sysadmining first.
If you install from sources read the admin guide (i.e., http://www.postgresql.org/devel-corner/docs/admin/installation.htm)
