必要 (ひつよう)

Necessary
～必要だ

必要(ひつよう) Is a な Adjective that means necessary.

• 水は必要だ – Water is necessary.
• 必要なお金をかします – I’ll lend you the necessary money.

いる

いる is a verb that means necessary, just like 必要。It’s a state, so it uses が。While verbs can modify nouns like adjectives, 必要 is typically used instead of いる for modification purposes. 

• 金がいる – Money is needed.
• 時間がもっといる – Time is (more) needed. (More time is needed). 

に + 必要

Necessary for Noun
Nounに必要だ

You can use に with 必要/いる to mark what something is needed for. The thing that needs something has a high probability of becoming the topic as well, so には is fairly common. 

• ウェブサイトにはお金がいる – Money is necessary for a website.
• パーティには人は必要だ。- People are needed for a party. 

*には here is just 「Xに」 being made the topic.

のに

Needed for Verb
Dictionary form verb + の + に

You can also combine the nominalizing の with the に above to mark an action as requiring something.

• 泳ぐのに水着は必要だ – A swimsuit is needed for swimming.
• 料理するのに火は必要だ。- Fire is necessary for cooking.
• ウエブサイトを作るのに金が必要だ – Money is needed to make a website.