Generally, you can just melt the chocolate over hot water, let it cool to about body temperature, and then use it in your recipe.
Most recipes call for unsweetened chocolate, also known as baker's chocolate or cacao mass. If you use milk chocolate or other chocolate bars for eating, you will have to adjust your recipe. Even so, the results will invariably be different from the original recipe, mainly heavier and less chocolaty.
As far as I know, milk chocolate can have as little as 10% or 15% chocolate, and about the same amount of milk.
An average chocolate bar might be between about 25% to 50% chocolate, most of the rest being cocoa butter and sugar. So, to get the same chocolate taste, you would have to increase the amount of chocolate and reduce the sugar.
Some chocolate bars have only about 25% sugar or even 15% or 10%. try to find out and do the math yourself.
My personal recommendation would be that, if you can't get baker's chocolate, try to find a chocolate of at least 80% chocolate and avoid milk chocolate.
It would help if you could post your recipe, especially if you have any more questions.