Saying "I want (something)" is pretty easy. Just say the thing you want and add ga hoshii to it.
のみもの が ほしい です。
nomimono ga hoshii desu.
(I) want a drink.
NOTE: The desu is optional and is usually dropped. nomimono ga hoshii. is perfectly fine in spoken Japanese.
Next, let's ask a question. Can you figure out how to do it? That's right add a ka REVIEW HERE
ケーキ が ほしい です か？
ke-ki ga hoshii desu ka?
Do you want cake?