Gothic II is a largely non-linear game. This walkthrough is written in an order that will result in you finishing all the quests possible before committing yourself to a faction, but most quests can be done at any time and in any order you like as long you have reached or passed the chapter in which they are introduced, or joined the particular guild if they require membership.
If this is your first time playing Gothic II it's probably a good idea to hold off installing Night of the Raven; while it does add many new items and Gothic cameos to the game it greatly increases the difficulty of enemies and makes character development slower. Once you have a good feel for the game or have finished the original feel free to try Night of the Raven. If you want to have both installed simply make a copy of your Gothic II directory before installing Night of the Raven. Note that Gothic II saves don't work in Night of the Raven and vice versa.