Unknown charge on my credit report. How do I get it removed.

Hi, my name is Bart and I was checking my credit at  Identity Guard.com and and
I found a charge that I do not recall doing at all this is the information:

Account Name: CITI
Account #:  xxxxxx
Account Type: Credit Card – Revolving Terms
Balance:  $6700.00
Date Opened: 3/1/1999

Mailing Address:
PO BOX 000
Jump off, SD 57117
How can I first find out what is this charge all about, and how can I fix
it?

I really appreciate your help, thank you.

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you write to the mailing address
and to Equifax Transunion Experian Equifax credit bureau
use this dispute form
and check your account