Assumed, I hash the next bitcoin block. Whom do I tell if it's decentralized? The bitcoin network is decentralized, right?

Assumed, I do hash a little bit and get a hash that starts with 20 Zeros..

What do I do than? I need to send the block to someone but who is that? It's decentralized... Do I send it to all the other miners?
You don't tell a specific node on the network, but rather broadcast your block to the entire network.  You essentially scream it from the rooftop.

If it's valid, it will be in the best interest of everyone to recognize your block and start mining on top of it.