Possession and attributes with の

の is a particle. It's a possessive marker and an attributive marker. "Attributive" here means that something is an attribute of something else, like an attribute in a video game, like the "flame" in "flame sword". The relationship is always straightforward.

私のネコ
My cat

君のX
Your X.

ネコの先
Ahead of the cat

男の子
Boy (metaphorically, male child)

If you get lost, the easiest way to remember の is: It's like "of", but it applies backwards.

夜の人
Person of the night

Sometimes "of" gets confusing, and you have to remember the idea of "attribute".

夜の海   
The night sea

海の夜   
Night on the sea (NOT "night of the sea")

の is literally the single most common word in all of japanese, so make sure you remember it well!

私の子供のネコが多い  
My child's cats are many. (My kid has a lot of cats.)