In Japanese saying you want to do something is different from wanting a noun. To change a verb into the want form, you add any of the following endings to the masu stem of a verb.
Hayashi wants to eat tempura = hayashi wa tenpura o tabetai
Wanted to read = yomitakatta (読みたかった)
Didn't want to go to hot spring = onsen ni ikitakunakatta
To make this more polite you add desu to the end.
Do you want to eat something? = nanika tabetai desu ka
Another way that you can say you want to do something, or ask someone if they want to do something is using the plain form of a verb. This can either mean one will or wants to do something.
(Do you want to/will you) drink this? = kore nomu? (これ飲む？)
(Do you wanna/shall we/will you) go to hot springs? = onsen iku?
I want it
When you are talking about wanting a object you need to use the adjective hoshii (欲しい). Below are the different ways you can conjugate hoshii.
I want cookie = watashi wa kukkii ga hoshii (私はクッキーが欲しい)
(I) want banana = banana ga hoshii (バナナが欲しい)
To express more politeness you can use desu and change the endings like other adjectives. When you just want to state you don't want something you use the particle wa instead of ga.
(I) don't want coffee = koohii wa hoshiku arimasen
(She) doesn't want banana = banana wa hoshiku nai desu
If you use the particle ga in the negative like the sentences above, it changes it meaning slightly. It gives a sense that you don't want something, (but you want something else).
Another way that you can express that you want something is using the verb iru which is commonly used ask if you want something. Such questions are very commonly asked in situations like the following.
(Do you) want this? = kore iru? (これいる？)
Don't want = iranai (いらない)
Technically speaking, the verb iru means to need and is used to express that you need something the same you would in English. It is also used to ask if you want something. In English this is expressed using separate words, but in Japanese it is the same word.
(I) need glasses = megane ga iru (めがねがいる)
Do you need a box? = hako ga irimasu ka (箱がいりますか)
(I) don't need (polite) = iranai desu (いらないです)
Would you like
To ask someone in a formal situation if someone wants something, or to ask someone how they feel about something you can use the word ikaga. This is only used in polite situations.
Would you like water = mizu wa ikaga desu ka
Would you like tea = ocha wa ikaga desu ka