Get Data grouped and ordered in a specific Structure out of Flat Sharepoint-Listdata

so I have the following problem:

I have a Sharepoint2013- CustomList containing multiple HelpContent-Entries. Entries are basically in the following structure:

ID||Category||Subject||Headline||Description||ImageUrl||SlideNumber

1||Week||WeekSubject1||Head1||Desc1||url1||1

2||Week||WeekSubject1||Head2||Desc2||url2||2

3||Day||DaySubject1||Head1||Desc1||url1||1

4||Day||DaySubject2||Head1||Desc1||url1||1

5||Print||PrintSubject1||Head1||Desc1||url1||1 ...(and so on).

Now I'm in need to get these flat Structure grouped and ordered into a hierarchical object-structure, to present it as json-result via a WCF-Service in a .NET-Application.

My preferred Structure is shown in the picture below:

So here I'm actually beginning to get problems.

More specifically: What's the best way to sort and order my SPListItemCollection as shown in the yellow box in the picture? It would be great to get some answers, I'm really struggling here (Haven't been in the .net/sharepoint-universe for the last two years).

Best Regards, Dominik