You did not specify which device you have so I will tell you how to set a specific ringtone on my device, a Blackberry Tour.
Go to Contacts and click the contact you want to change.
Press the Menu key and click Edit.
Scroll down to Custom Ring Tones/Alerts
Click Phone (this will change the ringtone when that person calls you)
Highlight Ring Tone: and select your new tone
Click Try It to test the ring tone to see if it is the one you like.
Press the Menu key
You can change the Message option too to change the tone that plays when that person sends you a SMS message.
I hope this helps,