Learn Polish: Jutro lecę do Paryża. - Tomorrow I am flying to Paris.

This sentence uses lecę (from the verb lecieć) because it describes a single, one-way trip to a specific destination. Lecieć is a unidirectional motion verb. You would use the multidirectional verb latać (latam) if you were talking about flying frequently, flying as a habit, or flying around without a specific destination.

Since jutro means tomorrow, why is the verb lecę in the present tense instead of the future tense?

Just like in English (where you can say 'I am flying to Paris tomorrow'), Polish often uses the present tense of motion verbs to talk about a scheduled, definite action happening in the near future.

Why does Paryż (Paris) change to Paryża at the end of the sentence?

The preposition do (to) always requires the noun that follows it to take the Genitive case. For most masculine inanimate nouns like cities, the Genitive ending is -a, which turns Paryż into Paryża.

Could I use the preposition w (in/into) or na (on/to) instead of do?