You usually connect USB cables to a USB port either on the back or the front of your case. I presume you have a modem that needs to be connected via USB. There are cards that can be put in a vacant slot inside the case, however as you do not appear to have any ports present it may be wise to see if the computer will be able to support a card before obtaining one. If you describe your computer a bit on here I am sure somebody will be able to tell you.
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