There are many places to start. Let me pick one simple element you may not have thought of: domain name. A domain name isn't much, ranging between about $7.00 USD and $30.00 USD for a year's worth. If you pick a regular domain you only have to pay by the year; some special, esoteric domains like .tv require many years of prepay. So there is that small expense. You can often go through your webhost for this and, in that case, they'll do all the technical setup for you. If you do it yourself at a registrar of domains you'll have to alter a few settings when you first buy hosting. This is not hard and your host will be happy to help you if they have any decency at all.
Next, a bit about hosting. There are many flavors. I'll stick to the ones you'd be likely to consider. The highest is a managed dedicated server, where you have an entire server to yourself and can use it as you will. Being managed, the host will take care of security upgrades and other issues you might not want to deal with. This service comes with increased cost.
Consider the TMF again. We're on a dedicated server but it is one managed by Jeff. The host he rents from does little to no work on it, only coming in when called upon for assistance. Usually you have a limit on how much time they'll help you for free per month. Prices for this kind of unmanaged server range from $50 on the low end to thousands on the high end per month. A managed server will be more. Very high traffic sites use these options.
A step below dedicated servers are virtual private server (VPS). This is where a server is partitioned into multiple miniservers, each customer getting a set fraction of the resources of the full server, usually a big fraction (a fifth or more). A high traffic site or one with resource-intensive tasks that don't quite mandate a dedicated server use VPS.
Below VPS is Reseller hosting. These are plans which are designed to be partitioned up and resold by the person who owns the account, letting someone run a web hosting business of their own without the investment in servers and such. Nothing bars you from using it for just your own account(s). They typically have a good amount of space and bandwidth but are subject to much more stringent resource usage limitations than VPS or dedicated servers.
Shared hosting is the bottom rung. These plans often advertise "unlimited" disk space or bandwidth to attract customers. In reality, using all that space would use up so much in the way of server resources that you would be quickly kicked off the server. This is what you'll likely want for a light-traffic site.
The best way to understand these "resource usage" things I've been tossing around is to read Terms of Service and Acceptable Use Policies at some hosts.
Here are three, two from hosts I use and one from Dreamhost, a well-known host I don't use. If you don't understand a particular element from one of those let me know and I can try to explain it.
Now, hosting types out of the way, let's consider those resources. First, disk space. MP3s can be made small while keeping decent quality; unless you think you'll have legions of users uploading you'll be fine. The posts themselves are irrelevant. To give perspective, the entire TMF database of posts and users takes up under 2 gigabytes of space. It's the attachments, at nearly 40 (if not more by now) gigabytes that really require space.
Connections are also crucial. Many shared hosts limit you to under 20 database/webserver connections at a time. That means twenty users doing something, be it browsing, posting or downloading a file. That does *not* mean online users. A user sitting and reading a thread does not count as doing something until they perform an action. If you're expecting a forum where you'll have huge amounts of traffic you need to consider a more expensive hosting option than the generic shared hosting services of most companies.
Bandwidth usage is unlikely to give you trouble. Most any host today (and I can direct you to a few if you like) will give you more bandwidth than you can shake a stick at. What you'll run in to before hitting bandwidth limits are the resource limits I mentioned above. In order to utilize all your bandwidth you'd need to use so much in the way of server resources that you could not survive with shared hosting.
You'll need to make certain your host has certain server technologies available to you to run vBulletin, namely PHP and MySQL. I can;t think of any reputable host that doesn't so this is not too much of a concern.
Customer support is a crucial element many forget. Is the support team at your host willing to help you with things outside of "the server is down" or "I can't find my bill" issues? If you screw up your forum will they be willing to lend assistance (although vBulletin is the one to contact for that)? A larger host like Dreamhost or Surpass Hosting (one of the two I use) will have more tech support folks active to help you. A smaller host like InfluxHost (the other I use) may see hours between ticket responses due to their smaller size and may not be as willing to hold your hand and guide you through issues. Then again, a larger host may be totally aloof from your issues and refuse to lend any real aid.
One thing to consider is paying month-to-month at a host for a little while, seeing if they are the right one for you before committing to the cheaper, but more risky, option of paying a year or more in advance. If you have your domain registered at an external registrar it is easy to move away from an uncooperative host. If you register through your host you may be trapped.
It is a lot to consider and I'm happy to keep discussing this if you have other questions, either raised from my gobbledygook or from elsewhere.
