Following is a walkthrough for Mass Effect 3. At the top of every page will be a video detailing the collectibles in a given level with a detailed written walkthrough. Side missions SHOULD be done as soon as they are available to avoid them failing automatically however the Side Missions are in a section following the main missions. All side missions are optional but will give you new gear, some War Assets and extra experience.
Following the side missions will be a walk through for all N7 Missions. These are generally much shorter than the side missions but give decent boosts or items to finish side missions. They are also required for an achievement/trophy.