If you set aside half a day or so to season that new smoker you can learn a lot about how well it retains heat and how to adjust the vents. When I had an offset I used charcoal for fuel (Kingsford was my preference) and added wood chunks for the smoke flavor. The easiest way I found to light the coals was with a chimney... I'd get one going good, put it in the smoker, and immidiately start the second. Periodically I'd start another chimney and add it when more fuel was needed.
Most say to season a new smoker at high heat... I'd agree, but after an hour or two I'd practice closing down the air vent(s) to get the temp down to 225* or so. Once there see how long it will stay on it's own, and add fuel when it begins to drop. It won't take long to get a feel for it.
I would suggest leaving both doors closed as much as possible. On of the secrets to good temp control is to let the smoker do it's thing without any 'help'.
Once you have that part behind you there are plenty of step by step instructions here on smoking a butt. Since this will be your first I'd suggest going with the simplest method, saving the fancy stuff for another time. Butts are very forgiving... if you put a basic rub on one and smoke it at between 225* and 250* until it reaches an IT of 200* to 205*, you will have a very tasty piece of meat. In order to have plenty of time I'd count on a cook time of 2 hours per pound.
Good luck. Let us know how it turns out!