I need to know from the very beginning about how I start.
closed as not a real question by underdark♦ Apr 10 at 20:19
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Sewers are much more often driven by hydraulic considerations, which includes elevations in the z axis. The spatial layout is a close second, which deals with coordinates in the x y axis. So what are the hydraulic constraints going to be? The selection of a Mannings n is by far the most troublesome variable.
Having said that, I note that there is a hydraulic plug-in for QGIS. In the long run, you would learn a lot by exploring QGIS and the G-Hydraulic plug-in. In the short term you ought to concentrate on your assignment. Many of the hydraulic issues for a sewer design could theoretically be dealt with without many x y coordinates. There are enough pipeline design packages out there which operate in the z dimension which would deal with elementary sewer design if you input your own Mannings n.
Perhaps you just want a map? By just drawing a diagram you do not develop the hydraulic model based on the xy locations of features. CAD software and GIS software with plugins will automatically include the spatial separation of features in the calculations which make up the model. For a diagram / map only I'd strongly recommend Google Sketchup.
So you should start by gathering the available data in the question. If the examiner did not supply x y data I would not embark on GIS activities.