What is the ruling on my action, when I was young, of writing the first letter of my name with fire on my hand, out of ignorance on my part, and what should I do now, as Allah has guided me?

This action is forbidden because it involves torturing and harming oneself. Ibn Hazm transmitted a consensus that it is not permissible for anyone to kill themselves, or amputate a limb, or harm themselves, except in medical treatment by amputating an ailing limb. Also, torture by fire is forbidden. The fact that the action was for the purpose of writing the first letter of your name does not change the ruling; it is a futile act with no benefit. However, there is no sin upon you, Allah willing, if you did this before puberty; for the pen is lifted from the child until he reaches puberty.