Let me try to explain it in simple terms.

The job of any web hosting provider is just to give you some space on the Internet (most commonly represented under your domain name) and tools to control it (such as create email addresses, databases, manage files, perform backups, view visitor statistics, etc). What you display in that space, which is basically your website, is totally up to you, as long as your content does not break any laws.

From the sound of it, you want people to first land on your website and then have a CHOICE to either read the regular website pages or click and visit a FORUM which is also installed. This is possible with EVERY web hosting provider. The difference is - some providers, as SwarmHost mentioned above, offer one-click forum application install as part of their "Fantastico" package (which is an add-on feature of cPanel control panel - a popular website management tool a lot of hosting providers offer).

However, that "Fantastico" package is limited to automatic installation of phpBB.com forum software on your website (in any "category/directory" or website location you want). If you'd like to have complete control over the type of forum software you want to work with, then my suggestion is first to go over forum applications I mentioned above, pick one you like the most, then find a web hosting provider who fits your needs and budget. ANY provider is able to offer you the ability to host a forum application.

The actual organization of your website is totally up to you and how you design/build your site to be. Whether you want people to land on your home page first, then go to your forum is in your hands alone.