What was Prince Charles first name?

HRH Prince Charles of Wales full name (without any titles) is Charles Philip Arthur George. None of the royal family members have a birth surname; however, they use "Windsor" as a surname to sign official and/or governmental documentation that requires a surname.

HRH Prince Charles of Wales full name (with titles) is His Royal Highness The Prince Charles Philip Arthur George, Prince of Wales and Earl of Chester, Duke of Cornwall, Duke of Rothesay, Earl of Carrick, Baron of Renfrew, Lord of the Isles, Prince and Great Steward of Scotland, Royal Knight Companion of the Most Noble Order of the Garter, Royal Knight Companion of the Most Ancient and Most Noble Order of the Thistle, Knight Grand Cross of the Most Honourable Order of the Bath, Member of the Order of Merit, Knight of the Order of Australia, Companion of the Queen's Service Order, Member of Her Majesty's Most Honourable Privy Council, Aide-de-Camp to Her Majesty.

To date, he has not officially chosen a regnal name for when he succeeds the throne.