Nokia 5800 XpressMusic - How can I set a song as a ringing tone?

You can set a song as ringing tone by selecting Menu > Music (or Menu > Music > Music player) and browsing to the desired song. Select Options > Use tone > Set as ringing tone (or As ringing tone). Select then the profiles for which the song will be used as the ringing tone.

If you want to assign the song as a ringing tone for a specific contact, select Options > Use tone > Assign to contact and select the contact from the list.