How to make a hot chocolate from a chocolate bar?

To clarify, my skills are on the level of putting butter on toast, or puting rice on a microwave.

What chocolate do I use? Milk or Dark?

What is the method and recipe?

How much sugar do I put?

How to heat the chocolate? Oven?

^{P.S. Do I use this subreddit, or there is a different one for this?}

bon appetit