Well it depends on your phone... but if your phone is compatible with the file type it's not a difficult process...and you've already got the usb cable... What model/manufacturer of phone do you have first of all?

Here's a link that should have everything you need...
http://www.nextel.com/support/faq/ringtonesfaq.shtml

I have a ringtone I d/l from the internet. I was wondering how would I get it to my phone. I have the Data cable I am wondering if this is possible. Also How would I get all these tones that people Post. Nextel seems to be the hardest phones to get tones for Thanks Adam