The whole point of setting the master key to 0 is to guarantee that you won't be able to make new transaction with the account. This proves that the configuration and amount of an asset won't change in the futur. This is true only when the account have no alternative signer.
Unfortunately, this means you won't be able to change anything to the issuing account anymore. If you're actually discovering Stellar possibilities with assets, I suggest you to keep your master key to 1 so you can change and experiment with it. Once you know more about how it works you'll be able to make the right setup from start and to cut the account if needed. This is not mandatory, though, as in some cases it may be desirable to still control the issuing account for further asset emition/setup like trust lines.
I hope it answers your question,