Learn German: Was ist dein Sternzeichen? - What is your zodiac sign?

Sternzeichen is a neuter noun (das Sternzeichen). In sentences using the verb sein (to be), the noun is in the nominative case. The nominative neuter form for "your" is just dein, with no extra ending.

How is the word Sternzeichen built?

It is a compound noun made of two simpler words: der Stern (the star) and das Zeichen (the sign or symbol). The gender of a German compound noun always comes from the last word in it, which is why it is das Sternzeichen.

Could I ask Welches ist dein Sternzeichen? instead of using Was?

Yes, you can! Welches (Which) is grammatically very precise because you are asking about a specific choice from a set of twelve signs. However, Was (What) is extremely common in everyday spoken German for this exact question.

This sentence is in the present tense. Why is it in a lesson about the future tense (Futur I)?

Asking someone's star sign is the perfect setup for reading a horoscope (das Horoskop). Horoscopes are a classic way to practice making predictions about what will happen, which perfectly uses the werden + infinitive structure you are learning now (for example: Du wirst heute Glück haben - You will have luck today).