1. Der Genitiv-Genitive Case
genitive case is used in German to express either:
• possession, ownership, belonging to or with:
-Hier ist das Auto meines Vaters. Here is my father’s car.
-Hast du die Freunde meiner Schwester gesehen? Did you see my sister’s friends?
• “of” in English, when referring to a part or component of something else:
-Am Anfang des Kurses haben wir viel gelernt. We learned a lot at the beginning of the course.
-Manche Seiten des Buches fehlen. Some pages of the book are missing.
• Genitiv prepositions: anstatt (statt) -- instead of, außerhalb -- outside of, innerhalb -- inside of, within, trotz -- in spite of, während -- during, in the course of, wegen -- because of:

2. Wer-who (nom)
Wen-whom (akk)
Wem-whom (dat)
Wessen-whose (gen)
Wessen Auto ist das?
Whose car is that?
Nom
Akk
Dat
Gen
Mas
der
den
dem
Fem
die
Neutral
das PL
den + n
des
der
des
der
German adds an ending to the mas/ neutral noun (person/place/ thing) in the genitive case. Add -es for single-syllable -s for 2 or more syllablesThere is no ending for the fem or the pl!
What is a syllable: Clap your hands to figure out the syllables.
Mann is short and you should be able to clap your hands once. Try it now 
Your ending will be des Mannes
Sommer is long and you should be able to clap your hands 2 times. Try it now 
Your ending will be des Sommers
masc neut fem pl
des Mannes des Buches der Frau der Blumen
meines Mannes meines Buches meiner Frau meiner Blumen

3. Word of warning:
Your impulse may be to simply put an -s before a noun to indicate the possessive, as we do in English (my father’s car). However, saying “mein Vaters Wagen” is not only incorrect in German, it is incomprehensible and makes no sense at all. You must rephrase: “der Wagen meines Vaters”. Think of it as saying “the car of my father”.
With personal names, you just add an -s to the person’s name to indicate possessive (Just like English without the apostrophe). If the name ends in a –s then add a -‘ after it like in English. But when referring to any other noun rather than a proper name, the genitive formation must be used:
Erins Freund heißt Thomas. Der Freund meiner Schwester heißt Thomas.
The friend of my sister is called Thomas.
Hans’ Mutter ist nett. Die Mutter meines Freundes ist nett.
The mother of my friend is nice.
Colorados Hauptstadt ist Denver. Die Hauptstadt dieses Bundeslands ist Denver.
The capital of this state is Denver.
Alternate method: We are not focusing on this, but it is important to know
The genitive case has been disappearing in German, but it’s not ‘dead’ yet. You won’t hear it in informal situations -- it’s reserved for formal writing or elevated styles of speech. Instead of the genitive to indicate possession, you will often hear the dative used with the preposition ‘von’:
das Haus meines Freundes = das Haus von meinem
Freund der Onkel meiner Mutter = der Onkel von meiner Mutter
die Namen der Kinder = die Namen von den Kindern