Learn Italian: Questo ristorante è nuovo, ma è molto famoso. - This restaurant is new, but it is very famous.

Ristorante is a masculine singular noun. Nouns ending in -e can be masculine or feminine, so you just have to learn their gender. Since it is masculine singular, the regular adjectives describing it (nuovo and famoso) must take the standard masculine singular ending, which is -o.

How can I easily tell that ristorante is masculine in this sentence?

The other words give it away! The word for 'this' is questo (masculine singular), and the adjectives nuovo and famoso both end in -o, which proves the noun they describe is masculine.

If this sentence was about a feminine noun like pizzeria, would molto change to molta?

No! In this sentence, molto means 'very' and describes the adjective famoso. Words that describe verbs or adjectives are called adverbs, and they never change their endings. If it was a pizzeria, you would say è molto famosa.

Why does the second half just say ma è instead of using a word for 'it is'?