What have you read so far, Volls?

Probably his most widely-recognized "Cthulhu" stories are:

The Call of Cthulhu
The Dunwich Horror
The Whisperer in Darkness
The Shadow over Innsmouth
The Haunter of the Dark
The Shadow Out of Time
At the Mountains of Madness

You could technically file a lot more of his works under the Cthulhu category, but the ones I listed are the stories that most-heavily incorporate the core values of the Mythos.

Other stories like "The Dreams in the Witch House", "From Beyond", "The Colour Out of Space" and "The Case of Charles Dexter Ward" incorporate many of the core values of the Cthulhu Mythos. But, at the same time, most of them also draw heavily on Lovecraft's obsession with the nature and potency of dreams, and could also fit into his "Dream Cycle" category.

In general, just read all of it. You'll find your favorites, and it's all really thought-provoking stuff.