You can use either particle (は or が) for the sentence, but there is a slight difference in nuance.
1)あなたの くるまは あたらしいですか。(ANATA NO KURUMA WA ATARASHII DESUKA) = Is your car new?
2)あなたの くるまが あたらしいですか。(ANATA NO KURUMA GA ATARASHII DESUKA) = Is your car new?
As for #1, this is the sentence pattern that you have learned in lesson 2.
[Topic] は [question or description]。
(Example 1) A: あなたは くるまが ありますか。(ANATA WA KURUMA GA ARIMASUKA) = Do you have a car?
B: はい。わたしのくるまは、ニッサンです。(HAI. WATASHI NO KURUMA WA NISSAN DESU) = Yes, my car is a Nissan.
A: あなたの くるまは あたらしいですか。(ANATA NO KURUMA WA ATARASHII DESUKA) = Is your car new?
は is normally used when specifying one object over all other objects or singling it out. The conversation is already in progress and A and B were talking about "B's car". Therefore, は is used in the third sentence.
You can use が instead of は when you want to stress あなたのくるま (your car) as in #2.
(Example 2) Ken and his friend Takeshi are looking at Ken's car and his father's car parking in the driveway. Takeshi thinks his father's car is clearer and newer than Ken's. Ken's car looks dirty and old.
Takeshi: おとうさんの くるまは きれいですね。 (OTOUSAN NO KURUMA WA KIREI DESU NE) = Your father's car is clean.
Ken: はい。でも、おとうさんのは、ふるいです。ぼくのが あたらしいです。 (HAI. DEMO, OTOUSAN-NO WA FURUI DESU. BOKUNO GA ATARASHII DESU) = Yes, but my father's is old. "Mine" is new.
Takeshi: え？ あなたの くるまが あたらしいですか。 (ANATA WA KURUMA GA ATARASHII DESUKA) = What? Is "your car" new??? (I can't believe it.)
You can change the particle (は or が) depending on the situation as above.
It takes time to get used to the two particles. You can use the sentence pattern from lesson 2 for now and then practice making (and reading) sentences using both particles. You will get it :)