I have a site that I am going to put up using a forum software PhpBB. But I hear this is a very unsecure software. Does anyone have any suggestions on how I can make this software more secure and or another free to low cost forum software that I can use to avoid being hacked.

If you use an open source script, people will inevitably find vulnerabilities. The best you can do is stay up to date with the latest version, run mod_suphp to limit damages, and use mod_security.

Actually, I disagree with mod_security wholly, as, in addition to being an extreme resource hog under heavy traffic, it causes more problems than it helps. To me, it's a geek tool --- and a lazy one, at that! It's simply not something that should ever be relied upon given the fact that if everything else is tightened up, it's completely useless and you're only adding another layer of complexity when trying to sort out why some other bit of software doesn't work.

Keeping up-to-date with the software in question, especially Forum and Blog software, is a major thing. But shoring up Apache & /etc/php.ini is where most people get into trouble --- I wrote the HOWTO here several years ago detailing the best ways to fix php.ini to avoid SQL Injection and Remote Inclusion hacks. And even with age on it, it's still valuable, and viable, information.

You're also a proactive administrator. For most users, it might be a good idea to install mod_security, and hope to block some of the more common types of vulnerabilities. Mod_security can become quite restrictive, depending on the ruleset you choose(or make). I have seen some servers where mod_sec is a complete memory hog, then others with tons more traffic, and it works just fine. It isn't great, but why not use it if it's available, and you can do so with minimal negative impact.