A basic form to express what I need something would be わたしは　XXが　ひつようです（ひつようだ）／いります（いる） ひつようです and いります goes with an object + が. But I think いります sounds more casual than ひつようです. Here are my translations: I need a pair of jeans. わたしは　ジーパンが　ひつようです。（わたしは　じーぱんが　いります。） I need a pencil. わたしは　えんぴつが　ひつようです。（わたしは　えんぴつが　いります。） If you'd like to say that you need to do something, a basic form would be this: わたしは + dictionary form of a verb + こと + が + ひつようです (We add こと after a dictionary form of a verb to make a noun phrase.) Here are examples: I need to study. わたしは　べんきょうすることが　ひつようです。 I need to go shopping. わたしは　かいものに　いくことが　ひつようです。 In this case, いります would not be used. I think it would sound awkward. I think these are enough for first learning how to express what you need. So, if you don't want to get confused, you can read the followings later. It's getting long, so I'll post another.

****** As I read again my translations, I feel these are stiff. I would say わたしは　べんきょうしないと　いけません instead of わたしは　べんきょうすることが　ひつようです. As Jay wrote in a comment, there's other expressions, ～なければいけません／～なければなりません and ～ないといけません／～ないとなりません, which are equivalent to "must" or "have to". (～なければいけません／～なければなりません are more of written words than ～ないといけません／ないとなりません.) The difference between ひつようです and ～ないといけません／～ないとなりません might be equivalent to the difference between "need to" and "must" or "have to", which I'm not sure about. Anyway, when you neutrally or objectively talk about what is necessary for some systems or procedures, ひつようです is more adequate: きっぷを　かうためには　まず　おかねを　いれることが　ひつようです。 In order to purchase a ticket, you first need to put coins into a machine. しけんを　うけるためには　しょるいを　ていしゅつすることが　ひつようです。 In order to take an exam, you need to send some documents. So, I feel more comfortable with わたしは　べんきょうしないと　いけません, because it's about something subjective. I hope this is helpful!

@Heidi I'm sorry for getting you confused. (^^;) I'll post my answer in an answering section. @Jay I'll answer your question in an answering section too.

Sorry guys I can't read kanji very well so im confused now

Ahh, I see. So, would this also be interchangeable with ～なければなりません／～なければいけません? Or is there a specific context where you'd use one over the other?