To express the desire “to want” we take the stem of the verb (i.e. the present polite form with ます cut off) and add たい. The verb now behaves like an i-adjective and conjugates in exactly the same manner.

Verb Group Dictionary Stem Want
to write 1 き〜 きたい
to read 1 み〜 みたい
to say 1 い〜 いたい
to see 2 たい
to do 3 する し〜 したい

And because we have effectively changed the verb into an i-adjective, we should use が instead of the object particle を for transitive verbs.

ビールがみたいです。
I want to drink beer.
それがしたかったですよ。
I wanted to do that.

That being said, you will often see the object particle used in both conversation and writing—as it was for the potential form of the verb.

景色けしきがとてもきれいですよね。写真しゃしんりたいです。
The scenery is incredibly beautiful, isn’t it? I want to take a picture.
残念ざんねんですね!一緒いっしょきたかったです。
That’s a shame! I wanted to go together.
上司じょうし出張しゅっちょうきたくないです。
I don’t want to go on a business trip with my boss.
今夜こんや六本木ろっぽんぎあそびたいです。
I want to hang out in Roppongi tonight.

To say want with a noun (e.g. “I want water”) we use the i-adjective ほしい.

あたらしい時計とけいしいす。
I want a new watch.
ものしい
I want a drink.

However, using ほしい is very direct—a child would use it to say, “I want water” or “I want something to eat”. Consequently, it is not appropriate to use to order in a restaurant, for example. We will see more polite ways of asking for something later when honorifics are introduced.