Learn Croatian: Učit ćeš hrvatski ovaj tjedan. - You will study Croatian this week.

This is a special rule for the Croatian future tense. When the main verb (like učiti, meaning 'to study') comes right before the future helper verb (in this case, ćeš), it drops its final -i.

Where is the word for 'you' in this sentence?

Croatian often leaves out subject pronouns like ti (you) because the helper verb ćeš already tells us exactly who is doing the action.

If I wanted to include the word ti (you), how would it change the sentence?

Adding ti changes the word order. The helper verb ćeš naturally wants to be the second word in the sentence, so it becomes: Ti ćeš učiti hrvatski ovaj tjedan. Notice that because učiti is no longer right in front of ćeš, its final -i comes back!

Why is there no preposition before ovaj tjedan (this week)?

In Croatian, you don't need a preposition to say 'this week'. You just use the phrase in the accusative case, which is used for time phrases. Because tjedan is a masculine, non-living noun, its accusative form looks exactly the same as its base dictionary form.