From https://wiki.debian.org/redmine
Install your database, e.g. postgresql (or don't, if you want to use sqlite)
apt install postgresql # or mysql-server
Install redmine with support for your database of choice
apt install redmine redmine-pgsql # or redmine-mysql, or redmine-sqlite
Database configuration can be fully automated, courtesy of dbconfig-common.
Configure web server
apt install apache2 libapache2-mod-passenger
cp /usr/share/doc/redmine/examples/apache2-passenger-host.conf \
/etc/apache2/sites-available/redmine.conf
edit /etc/apache2/sites-available/redmine.conf
a2enmod passenger
a2ensite redmine.conf
service apache2 reload
Using passenger (libapache2-mod-passenger) is usually the easiest way. For using in a domain shared with other applications, base your config on /usr/share/doc/redmine/examples/apache2-passenger-alias.conf instead.
You can then log into Redmine http://servername (ServerName that you put in /etc/apache2/sites-available/redmine.conf and in your DNS or hosts file) with the following default admin account:
username: admin password: admin