〜たい

To want to do something, put たい on the verb's ます-stem: 食べます → 食べたい, "want to eat"; 行きます → 行きたい, "want to go". Politely, add です: 行きたいです.

The result is an い-adjective, so it conjugates like one: 行きたくない (don't want to go), 行きたかった (wanted to go).

With たい, the object can take が instead of を: 水が飲みたい and 水を飲みたい are both fine.

Like ほしい, たい is for your own wants, or for asking someone else directly. Asking a superior 何を食べたいですか is too blunt; 何になさいますか is the polite version you'll hear in shops.