Let's back up a bit, what you are asking is a bit odd.
Speakers do not normally have a USB input, do yours? What model are they?
Do you have a receiver with a USB input? If so, the manual will tell you how to use it.
If you just happen to have a pair of speakers around for use with a computer, the only way you can use them without one is to hook them up to an amplifier that has a USB input. And you need to make sure the speakers can be used with an external amp, many have onboard amplification, so you may not be able to use them with an amp. Maybe a pre-amp and just run the output to the speakers without an amp.