Learn Italian: Non credo a questo ragazzo.

In English, you 'believe' a person, but in Italian, the verb credere (to believe) is followed by the preposition a (to) when you are talking about believing someone. Literally, you are saying 'I do not believe to this boy.'

Why does the sentence start with Non credo instead of Io non credo?

In Italian, it is very common to drop subject pronouns like io (I) because the ending of the verb already tells you who is doing the action. The -o ending in credo clearly means 'I believe', so the io is not needed.

Why is the word for 'this' questo here?

The word for 'this' has to match the gender and number of the noun it describes. Since ragazzo (boy) is a masculine, singular noun, you must use the masculine singular form, questo.

Where does the negative word non go in an Italian sentence?

Unlike in English where we use 'do not' or 'don't', in Italian you simply put the negative word non immediately before the verb it is negating. Therefore, 'I do not believe' is always Non credo.