Depending on how long it has been seized, it might be kinda easy, or a huge job. Start by removing the spark plug and spraying a large quantity of penetrating oil directly into the cylinder. Let it sit for a day and then try turning the crankshaft with a wrench. Don't try using the starter until you can turn it easily by hand. After you get it turning you can pour in a little motor oil and turn it some more. When it is easy to turn, let the starter spin it for a while. Put the spark plug back in and see if it will start. The biggest potential problem is that the rings might be "set" and it won't have good compression, and it might also burn oil. If that is the case, an engine rebuild will be in order.