Learn Portuguese (Portugal): Nós comemos um bolo delicioso.

In Portuguese, adjectives (words that describe a noun) usually come after the noun they describe. So instead of 'delicious cake', you say 'cake delicious' (bolo delicioso).

Why is it delicioso and not deliciosa?

Adjectives have to match the gender of the noun they describe. The word for cake, bolo, is a masculine noun (which we can also tell from the masculine word for 'a', um). So, the adjective takes its masculine ending: delicioso.

What is the dictionary form of comemos?

The verb is comer (to eat). The ending -emos tells us that 'we' (nós) are doing the action. Fun fact: comemos is exactly the same word for both the present tense ('we eat') and the past tense ('we ate')!

Do I have to say Nós, or can I just start with Comemos?

You can drop Nós! Because the verb comemos can only ever mean 'we eat', the pronoun is optional. Native speakers often leave it out unless they want to add special emphasis.