The symbol for "in care of" is "c/o." It is used when mailing a letter or package to someone at the address of another person. It is also sometimes used to send the mail to someone at a business address.
Correspondence sent "in care of" someone is addressed to the intended recipient with "c/o" under the name followed by the name of the person or company at the address to which the correspondence is to be delivered. Whether the mail is addressed by hand or on a computer, the sender simply writes or types a "c" followed by a forward slash and then an "o." On a computer, the "c/o" symbol may also be made by choosing "Insert" and then "Symbol" and typing in the code 2105.