How to say needs in japanese? I need (object), and I need to do something.?

Question:

I know using hitsuyou and iru say I need (blank), like kagi ga hitsuyou desu means I need a key and konpyu-ta- ga hitsuyou desu means I need a computer, but when do I use iru and when do I use hitsuyou? Would replacing hitsuyou with iru mean the same thing? Is one formal and informal?And how would I say I don't need a computer/key, I didn't need a computer/key, and I needed a computer/ key? ( Negative, past negative, and past tenses for both). I only have the present tense for hitsuyou down.

Now how would I say I need to do something? There is base 1 + nai to ikemasen and base 1 + nakucha ikemasen.
Tabenai to ikemasen ( I need to eat ) and yomanakucha ikemasen ( I need to read). Again, which do I use? Nai to ikemasen or Nakucha ikemasen? Is one formal and informal? For these also, how would I say, I don't need to eat/read, I didn't need to eat/read, and I needed to eat/read.( Negative, past negative, and past tenses for both). I only know how to use the present tense.
A simply put detailed explantion would be very nice please. Something simple that I can understand easily, and an actual explained answer, not an answer key. Thank you soo much for your consideration in answering and reading my question.

Answer:

The difference between hitsuyou and iru, is the definition. Hitsuyou means 'necessary, needed, essential,etc'. So you use it when it's really necessary.
Here are some example sentences:
歯医者に行く必要はない。
haisha ni iku hitsuyou wa nai
"You don't need to go to the dentist."

私たちは生きていくためには衣、食、住が必要です。
watashi tachi ha ikiteikutame ni wa koromo, shoku, juu ga hitsuyou desu.
"We need food, clothes, and a home in order to live."

For iru, you can use it when it's just a regular need.
Here is an example sentence
今のところお金は要らない。
ima no tokoro okane wa iranai.
"I don't need money right now."

and for the 'naitoikemasen' questions,
it's not 'naitoikemasen'. When you need to go something, for example, you need to go, you say
"ikanakerebanaranai.'
for eating: tabenakerebanaranai"

the slang way is just " nakucha" or "nakya"
example:
"Ikanakucha" "Ikanakya"
I have to go.

I don't really know how to put it in negative form. I think it's
"nakerebanaranai ga arimasen/ de arimasen" but I'm not completely sure