No em crec la notícia que he llegit a internet.

Em is the first-person singular weak pronoun, and here it is part of the very common pronominal use of creure.

In this pattern, creure's alguna cosa means to believe / buy something as true. So No em crec la notícia means I don't believe the news story or I don't buy it.

It does not literally mean I don't believe myself in English. The pronoun is just part of how this verb is often used.

For example:

• M'ho crec = I believe it
• No t'ho creus? = You don't believe it?

Yes, often there is a difference in use.

A simple way to think about it:

• creure often means to believe, to think, or to believe someone
• creure's often means to believe something you hear/read, especially in the sense of accepting it as true

Compare:

• Crec que vindrà = I think he/she will come
• Et crec = I believe you
• No em crec aquesta història = I don't buy this story

In your sentence, the pronominal form sounds very natural because the speaker is reacting skeptically to a specific piece of information.

Because Catalan usually needs an article or another determiner with a singular count noun.

So Catalan normally says:

• la notícia
• una notícia
• aquesta notícia

Bare singular nouns like notícia are much less common than in English.

Here la notícia refers to a specific news item, and it is further identified by the relative clause que he llegit a internet. So la is the natural choice.

Here que is a relative pronoun. It introduces a clause that describes la notícia.

So:

• la notícia = the news item / the story
• que he llegit a internet = that I have read on the internet

Together:

• la notícia que he llegit a internet = the news story that I read on the internet

A useful point: que does not change for gender or number here. It stays que whether the noun is masculine, feminine, singular, or plural.

Because in Catalan, no normally goes before the weak pronoun(s) and the conjugated verb.

So the order is:

• No em crec
• No ho sé
• No la veig

This is the normal position in standard Catalan.

A helpful general rule:

• with a conjugated verb, weak pronouns usually go before the verb
• and in negative sentences, no goes before that whole verb group

He llegit is the present perfect, while vaig llegir is the Catalan periphrastic past.

In many contexts, both can be possible, but they are not felt exactly the same.

• he llegit often suggests something recent or something still connected to the present
• vaig llegir places the action more clearly in the past

In this sentence, he llegit sounds natural because the speaker is reacting now to something they have read online, and that reading still matters to the present situation.

So the sentence has a kind of I’ve just read this / I’ve read this and now I’m reacting to it feeling.

Because with the auxiliary haver, the past participle normally does not agree with the direct object in modern Catalan.

So you say:

• He llegit la notícia
• He llegit les notícies
• He llegit el llibre
• He llegit els llibres

In all of these, the participle stays llegit.

That is why que he llegit a internet is correct, even though notícia is feminine singular.

In standard Catalan, a internet is a common way to say on the internet or online.

So:

• llegir a internet = to read on the internet
• trobar a internet = to find on the internet

Usually, no article is used there, so a l'internet is not the normal standard choice.

You may also see per internet, but that often emphasizes the medium, meaning something like via the internet. In your sentence, a internet fits well because it means the place/source where the information was read.

It might be understood, but No em crec la notícia is much more idiomatic and natural for this meaning.

Why?

Because creure's is very commonly used when talking about whether you accept a story, excuse, rumor, or piece of news as true.

So:

• No em crec la notícia = I don’t buy the news story
• No em crec aquesta excusa = I don’t buy this excuse

Using plain creure is more common in patterns like:

• Crec que és veritat = I think it’s true
• No crec que vingui = I don’t think he/she is coming
• Et crec = I believe you

Not with the same meaning.

Creure en means to believe in in the sense of having faith in something, believing in its value, or believing in its existence.

For example:

• Crec en tu = I believe in you
• Crec en la democràcia = I believe in democracy
• No crec en fantasmes = I don’t believe in ghosts

But for a specific news item or claim, Catalan normally uses creure or, very often, creure's:

• No em crec la notícia

So No crec en la notícia would sound wrong or at least very unnatural for the idea in your sentence.