how to create a calculated column

I want to create a table, this table's name is "Prod", in his table, I have the: **ID**, **name**, **Buying_price** and **selling_price** when the user inserts the data, I want him to enter "name, and buying_price" and I'd like the column of **selling_price** to be calculated depending on the user's input by this method (buying_price + 20% Buying_price) I don't know how to do this, I thought that a function/procedure could help but I have no idea how to do it, since I am new to database and SQL I would so much appreciate any help