Well to me it's depending on if you want to learn or you need to get the forum added to your site soon.
If you want to learn, it's best to sit down and plan it out. Make sure you have access to a database(phpmyadmin), because you will need it. Just start coding piece by piece.
If you are in a hurry (Like your boss in on your back) you can't beat phpBB or vBulletin.. I am sure hundreds if not thousands of hours have went into development of those projects. You can get a solution up quick.. The only part you would have to worry about is customizing the look.
As far a great framework. There are many out there like yii framework, Cake, Zend.. You can google php frameworks and you will prob go through the results all day..Each one of the frameworks have a learning curve.
Hope this helps