Best look up recipes specifically for cooking cakes using a microwave oven (browse on line, take a book out of the library or you could buy one in a bookshop) because, as you are aware, conventional oven way of cooking is entirely different and would be difficult/impossible to transfer from one method to another using the same recipe.
If your microwave has got a convention function then usually it can be preheated, follow the instructions in the manual. Microwaves don't need preheating really, as it goes hot very quickly. Half the baking time given in the recipe, e.g. if it asks for 30 minutes, reduce it to 15 and if it doesn't seem to be done yet, continue on for a few minutes. I've got to experiment a bit with mine to. Hope that helps and happy baking!