Hello, I am Yoshi from NILS Japanese language school.
[Vます-form たいです]
1) Verb ます-form
The form of a verb when it is used with ます is called the ます-form. In the word かいます, かい is the ます-form.

2) Vます-form　たいです
This expresses the speaker’s desire to do something. It is also used to ask what the listener wants to do. In this expression, as is seen in the second example below, the particle が can replace the particle を. The other particles cannot be replaced by が. Vます-form たい inflects as an い-adjective.
(Example)
わたしは　日本へ　行きたいです。　　I want to go to Japan.
わたしは　てんぷらを（が）　食べたいです。　　I want to eat tempura.
神戸で　何を（が）　買いたいですか。　　—くつを（が）　買いたいです。
What do you want to buy in Kobe?　　—I want to buy a pair of shoes.

[Note 1] ほしいです or ～たいです cannot be used to descrive the third person’s desire.
[Note 2] You cannot neither use ほしいですか nor Vます-form たいですか when you offer something or invite someone to do something. For example, when you offer a cup of coffee or invite the listener to have a cup of coffee, you should not say, “コーヒーがほしいですか,” nor should you say “コーヒーをのみたいですか.” Expressions such as “コーヒーはいかがですか” or “コーヒーをのみませんか” should be used.

August 29th News, from NILS Japanese language school.