How to decrypt AES in CBC i am having problems understanding how CBC works! I know that it is a chain and the IV is XORed with the PT but then what? I mean we encrypt it with key k and the result is the CT which we use for the next chain-XOR but my question is HOW the encryption works? For example in Stream ciphers i just had to XOR the K with the M (or C) to get my result, but on block ciphers i am completely lost...
CBC is a block cipher mode of operation, not a cipher. It can work basically with any block cipher - the choice of the block cipher itself is independent (to a large extent) from the choice of the mode of operation.

Examples of well known block ciphers include AES, DES, TripleDES, Serpent, Blowfish etc.

The title of your question, however, asks about decrypting AES using CBC mode operation. In which case you should be using AES, $P_t = \operatorname{AES-128-DEC}_k(C_t) \operatorname{XOR} C_{t-1}$. AES decryption is available in many crypto libraries (for example libtomcrypt, Crypto++, openssl, Java Cryptography Extensions, Python Cryptography Toolkit), and you can even use online AES "calculators" such as this one.