Im pretty new to bitcoins, so I may have a silly question. Imagine that someone has my public key. Can he find out how much bitcoin do I have? only using my public key?
Can he find out how much bitcoin do I have?
Yes, Bitcoin addresses are derived from public keys.
If someone has your public key, they can obtain your BTC address and therefore search the blockchain for transactions involving your wallet address. On the other hand, if you use a separate private/public key pair (separate address) per transaction and someone only has one of your public keys, they will not have enough information to obtain the address from the other keys unless you are transacting between them and it is somehow obvious that they all belong to you (analysis).
By knowing your public key, someone can easily obtain your BTC address and enter it into any block explorer website like blockchain.info and see your entire transaction history and current balance.