This article is from the Credit cards and Consumer Credit FAQ, by firstname.lastname@example.org (Steve Adams) with numerous contributions by others.
The instructions are printed on your bill, probably on the back.
Just follow them. The rules are simple: if you report a problem in
writing within 60 days of the billing date, the bank must
investigate it and respond to you within 30 days. While they are
investigating, you don't have to pay the disputed amount or any
finance charges on it. If their investigation shows the item was
correct, they can restore finance charges retroactively and you will
have to pay them.
The address to write to is on the bill. Look for a heading like "In
case of error" or "Send inquiries to."
Some banks try to resolve problems over the phone; others insist
that you write a letter. If you decide to call before writing, make
sure you note the date and time of the call, whom you talked to, and
what s/he promised to do (if anything). Then send a letter to the
"Send inquiries to" address mentioning this information. (Your
letter should make clear that you are confirming a telephone
conversation, so that the bank doesn't try twice to resolve the same
If you resolve a problem by phone, but the bank doesn't follow
through, the confirming letter that you sent will preserve your