Learn French: C'est un rendez-vous amoureux, alors sois naturel !

This is the imperative (command) form of the verb être (to be) for tu (informal "you"). The verb être is highly irregular, so instead of just using the normal present tense form (es), you have to memorize its special command form: sois.

Does un rendez-vous always mean a romantic date?

No, by itself, un rendez-vous just means an appointment or a meeting (like with a doctor, a client, or a friend). Adding the adjective amoureux (which means "in love" or "romantic") is what specifically turns it into a romantic date!

Why is the adjective naturel in the masculine form here?

In commands, you drop the subject pronoun (like tu), but any adjectives still have to agree with the person you are talking to! Because the sentence uses the masculine naturel instead of the feminine naturelle, we know the speaker is giving this advice to a man.

Could I use donc instead of alors to mean "so"?

Yes, you could definitely say donc sois naturel ! Both alors and donc can mean "so" or "therefore". alors is very common in everyday spoken French to introduce the logical next step or consequence of a situation.