Although many travellers set up their Simien trek through an agency in Gondar, it is also possible to make your own arrangements at the park headquarters in Debark, a process that usually takes a couple of hours. Guides, cooks, game scouts and (if required) mules can all be organized on the spot, ideally the afternoon before you want to start trekking.
As far as packing goes, you need to bring plenty of warm clothing, as it can get very chilly at night. You should also buy all the food you require for the duration of your trek in advance, either in Debark, or better still in Gondar or Addis Ababa. A good map, such as the Simen [sic] Mountains World Heritage Site sheet (University of Berne; 2003) will be useful too, especially if you plan on going it alone without a guide, though it may be difficult to locate on the ground. Most other equipment can be hired on the spot, either from the Simen Park Hotel or the park headquarters.