I don't have this particular phone, but I do have the search engines, so a bit of investigation revealed that Samsung has documentation online for all of its products, including a splendid user's guide for the SGH-D500 cellphone (you need to go into the customer download section to find it)

To turn an MP3 into a ringtone, you need to download the song onto your cellphone, as you've already done, then access the MP3 player screen, which looks like this:

Once you've found the song you want in the MP3 player, click on the Options button and you'll see a bunch of choices, most notably Set as. Choose that and you should be able to then select Set As Ringtone.

That's all there is to it. At least, theoretically. :-)

Go the Samsung site and download the documentation on your phone. It's very good and even designed to be best read on a computer screen, not a piece of paper.