That's a very open question. It's almost like asking "does anyone know how to create a website?".
Do you want to make a website that sends ringtones over a GSM server (SMS messages) directly to units? Or a website where people can download music using their WAP browsers? Do you want to charge people for the ringtones, or give them out for free?
If I upload a MIDI file onto any webspace I can access, go directly to that link from my mobile's WAP browser, and press 'Save' it becomes a ringtone. That's a very basic ringtone website.
Then you have the other end of the scale; popular sites with huge song databases updated every hour, premium text lines to request songs, immediate SMS delivery, online previews/payment gateways/credit systems, well designed WAP and UMTS (3G) optimised access portals; et-cetera.
What you want probably falls somewhere between those two extremes... But there's still alot to think about, and no way anyone can give a useful answer to your question without some more details.
Well. You'll need access to a GSM server; (you can hire access the same as you would to a HTTP server). You'll need a contract with an SMS service provider (which is often included with GSM server packages). You'll need a premium rate SMS number (I have no idea how you'd get one of those).
After sorting all that out you just need to make a website, you'll probably want a site for desktop pcs, and a site for mobiles (you don't really need to use WML for WAP anymore because alot of phones come with highspeed full content internet connections these days).
And then you need to get hold of your ringtones (you'll probably have to pay for them unless your making them yourself), categorise them intelligently and give every ringtone a unique ID, put previews on your desktop website where you advertise the ID and your premium SMS number (and work on ways to stop people like me downloading your previews and sending them direct to my phone). Then, write some code to run on your SMS server to relate a received ringtone ID code with a ringtone and send that ringtone in an MMS/SMS+attachment message back to the phone number that requested it.
I'm afraid I can't help you anymore than that. I've never done this; I was planning to do something similar at one point, and did an amount of research.