Learn Portuguese (Portugal): O molho de tomate está pronto.

We use está (from the verb estar) because being ready is a temporary condition. The sauce has not always been ready, and it will not be ready forever. If you used é (from ser), it would sound like being 'ready' is a permanent, defining characteristic of the sauce!

How do I pronounce molho here, and does it mean anything else?

In this sentence, molho means 'sauce' and the first 'o' is pronounced as a closed vowel, sounding somewhat like the 'o' in the English word 'boat'. Be careful, because there is an identical-looking word, molho, which means a 'bundle' or 'bunch' (like a bunch of keys), but that one is pronounced with an open 'o' sound, like the first sound in the English word 'awesome'.

Why is it molho de tomate instead of just molho tomate?

In Portuguese, you cannot stack nouns next to each other to create a new phrase like we do in English with 'tomato sauce'. Instead, you have to connect the two nouns with the preposition de (of). You are literally saying 'sauce of tomato'.

Why does the sentence end with pronto instead of pronta?