You need to use a verb stem together with たいです to describe what you want to do.

You can get a verb stem by removing ます from a verb in present affirmative form.

きょう　ハンバーガー　を　たべたいです。

I want to eat a hamburger today.

*たべ is a verb stem of たべる, eat. You can get たべ by removing ます from たべます, which is present affirmative of たべる.

*You can also say　きょう　ハンバーガー　が　たべたいです。

なつやすみ　は　きょうと　に　いきたいです。

I want to go to Kyoto on summer vacation.

きょう　は　はやく　ねたいです。

I want to go to bed early today.

きょう　は　ハンバーガー　を　たべたくないです。

I do not want to eat a hamburger today.

きのう　は　はやく　ねたかった　から、はやく　いえ　に　かえりました。

Because I wanted to go to bed early yesterday, I went back home early.

さとうさん　は　いえ　に　かえりたがっています。

It seems Mr. Sato wants to go home.

* When you want to describe someone else's wishes, you need to use a verb stem plus たがっています.

References:

I want something

https://anchor.fm/yuya-sato/episodes/I-want-something-epa75c

Verb Conjugation

https://anchor.fm/yuya-sato/episodes/Verb-Conjugation-emv716

Particles "wo", "de", "ni", and "he"

https://anchor.fm/yuya-sato/episodes/Particles-wo--de--ni--and-he-en1dme

Adjectives

https://anchor.fm/yuya-sato/episodes/Adjectives-en9o8v

Past Tense of Adjectives

https://anchor.fm/yuya-sato/episodes/Past-Tense-of-Adjectives-en9trh

Instagram:

https://www.instagram.com/yuyaman1986