Learn Portuguese (Portugal): Preciso de almoçar contigo amanhã.

In Portuguese, the preposition com (with) merges with certain pronouns to create special single words. When you want to say "with you" to someone you speak to informally, the word for "with" combines with the pronoun to become contigo. It is incorrect to say com tu.

Why do we need the word de after preciso?

The verb precisar (to need) requires the preposition de (of/about) to connect it to the thing you need or the action you need to do. Whenever you say "I need to do something", you will use a form of precisar followed by de and then the action word, just like almoçar here.

The English translation says "to have lunch", but I only see one word for it here (almoçar). Why is that?

In English we usually use an action and a noun ("have lunch" or "eat lunch"), but Portuguese has specific action words (verbs) for eating meals. Almoçar means "to eat lunch". You will also see this single-word pattern with dinner (jantar).

Where is the word for "I" (eu) in this sentence?