Whether you need something or need to do something, you need to know what words to use in Italian to express that need, especially since there's no simple, one-word equivalent of the verb "to need." In the following example, Marika uses a highly irregular verb that’s quite common in Italian, but which causes quite a bit of confusion for non-native speakers, both because it doesn't get conjugated and because it's so similar to its related noun form. It's practically useless to mention the infinitive because it doesn't ever get used.
Marika gives a news report about a school perched high on a hill. Let's see what she says:
Per arrivare nella scuola più piccola d' Europa bisogna fare trecentocinquanta scalini.
To get to the smallest school in Europe, you need to go up three hundred fifty steps.
Caption 3, Anna e Marika - in TG Yabla Italia e MeteoPlay Caption
In talking about the search for the right location for his restaurant, here's how Gualtiero Marchesi uses bisogna:
Bisognava inventarsi tutto.
I had to invent it all.Play Caption
The infinitive of this indispensable verb is bisognare, but you never see it in this form, nor in any conjugation except the third person, where it is used impersonally. Marchesi uses it in the past tense: bisognava, and it will often appear in the conditional (bisognerebbe) or the future (bisognerà) as well.
Bisogna is a quick and neutral (sometimes maddeningly neutral) way to talk about what needs doing. For example, one housemate might say to the other:
Bisogna comprare il pane. (One needs to buy bread.)
Clearly, bread needs to be bought, but who's going to buy it? That detail is left to our imagination (or sense of duty).
This statement can also easily be expressed in the conditional:
Bisognerebbe comprare il pane. (Someone should buy bread.)
or in the future:
Bisognerà comprare il pane. (Someone will have to buy bread.
This way of using bisogna is easy: bisogna + verb in the infinitive
The other and more complicated way is: bisogna + che + verb in the subjunctive, but that's a topic for a future lesson.
Another way to express need is with the related noun bisogno (need), which is also easy to use, since the only verb you need to conjugate is avere (to have).
In fact, Gualtiero could have said:
Avevo bisogno di inventarmi tutto. (I had need of inventing everything for myself.)
He also could have said:
C’era bisogno di inventarsi tutto. (There was need [it was necessary] to invent it all for oneself.)
This is also easy because the verb essere will always be in the third person singular. In the above example, it's in the simple past.
It all has to do with sorting out the difference between bisogna (verb) and bisogno (noun) and remembering the simple rules about how they work. For a full explanation see this article.
In a nutshell:
Putting them together just for fun:
Bisogna andare in banca. In effetti, c'era bisogno di andare ieri, ma ieri bisognava fare tante altre cose. Bisognerà anche andare a fare la spesa questo pomeriggio, quindi, di che cosa abbiamo bisogno? C'è bisogno di fare una lista. Avrei bisogno di caffè, ma per quello, bisognerà andare in un altro negozio. C'è bisogno dello zucchero? No, non ce n'è bisogno, anche perché ho bisogno di dimagrire. Bisogna vedere, però, se riesco a berlo amaro.
We/I need to go to the bank. Actually, it was necessary to go yesterday, but yesterday we needed to do lots of other things. We'll also need to go food shopping this afternoon, so what do we need? We need to make a list. I would need coffee, but for that I need to go to different store. Do we need sugar? No, I don't need any, because I need to lose weight. We'll have to see if I'm able to drink it bitter [with no sugar].
See how you can mix and match these ways of needing things, or needing to do things. Just keep in mind the way they work, and which is which.
Now that you know the ins and outs of bisogna and bisogno, do a Yabla search and see for yourself how often these words get used in speech. Bisogna solo fare pratica! (You just need to practice!)