Download the wallet, send coins to it, wait 8 hours and you'll start staking. If you already have coins in your wallet, you just need to leave it open (and unlocked).
How much coins do i need? And how will I know when I earned from staking?
Any amount > 0. You'll see how much you earn when one of your coin inputs stakes a block - you'll see a small incoming transaction as if you mined it.