You will generally get better results if you use butter, olive oil, or a like spray on the outside of the bread. If you butter/oil the pan, especially if done liberally, the bread will tend to absorb the oil and will result in a greasy mess of a sandwich. It will, however, still toast if you choose to use nothing at all, but will not have the additional flavor provided by the butter or will not produce even browning over the bread.
Assemble the entire sandwich first and then place into the pre-heated pan over medium heat. I recommend using a light spread of mayonnaise on the inside of the sandwich in contact with the cheese, as it helps to keep the inside from drying out and adds a nice flavor, IMO. You can also add tomato slices, thinly sliced meats, or pickles if you wish.
Use a medium heat and watch it closely - do not leave to do something else while it is toasting. Check the underside regularly to ensure it doesn't burn. Turn over when toast is evenly browned.
There is a restaurant near my house called "Nickson's" that makes a fantastic grilled cheese/beef brisket sandwich, and was featured on an episode of "Chicago's Best" on YouTube if you'd like to see it done professionally.