If you just want to choose, say, a music track then you should be able to do that directly from within the music player (select track, MENU, Set as Ringtone - might be slightly different depending on software).
If you want more control, download Ringdroid from the Android Market - let's you use anything (including a recording) as a ringtone, alarm, etc. and also allows you to trim so that you only get the bit you want.