Back when we were thinking about starting a new forum to replace Astronomyforum.net. We looked at a number of "Paid" and "Free" software to use. We decided on phpBB because of the many features and 'add-on' (aka extensions) plus the ability to add custom styles that members could select from.
phpBB is based on the php computer language which has been around for quite a while. The website for phpBB has a great set of instructions and it's pretty easy to do a search for specific information you might be looking for. Also by using Google and typing "phpBB" and your question you can find lots of help. Now a warning...... phpBB has been around for quite a few years and has gone through many upgrades. We attempt to use one of the most recent versions, so be careful that you are looking at information that is fairly current. There is quite a bit of old info that no longer works with the newer versions of phpBB. That's why we have a "TEST forum". Try it here first. If you crash it we can either fix it or restore it, if you play with the live forum it can also be restored but there is always a potential of a loss of information.
Here's a link to the phpBB website. https://www.phpbb.com/
