Check out Atlassian's page for Confluence System Requirements. If you had your choice of hardware/software, this is probably the "best" configuration:
Provide at least 2GB of RAM. Depending on how many users you have, other factors follow. See the hardware sizing guide for a better idea of what you need for how many users.
Ubuntu or CentOS. Lots of people use Windows, but if you have any *nix experience, go that direction.
Java runtime environment
Use the one that bundles with your Confluence install. If you go with Linux, you won't need to do any extra work for Java.
PostgreSQL is the Atlassian recommended database if you have your choice. MySQL comes in a close second with Oracle trailing in third and a "try to avoid" warning for MS SQL. The general wisdom is that you should choose whichever database you have support for regardless of the recommendations; if you have DBAs that know MS SQL but only MS SQL, having a DBA is better than not having a DBA.
Technically you don't need a webserver to frontend Tomcat for you, but it's best-practice. It's also highly recommended if you plan on securing your site with HTTPS - setting a reverse proxy up with HTTPS is a lot simpler than trying to secure Tomcat. Atlassian does not officially support reverse proxies (that is, support.atlassian.com will probably not help you fix yours if it is misconfigured) but they provide documentation for using both Apache and nginx. Personally, I recommend nginx as we have used both nginx 1.8 and Apache 2.4; the performance was much better with nginx.
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