The short answer is easy, because it's pretty much the same as English, altough there can be some differences.
- Necesitado = needed (adj.) or needy.
- Necesitar = need (v.)
- Necesario = neccesary (adj.)
So, if you want to express neccesity, it's directly:
Es necesario que [...]
Which means 'It's neccesary to [...]' or '[...] is neccesary / required'.
This sentence is very frequent. For example:
Es necesario actuar pronto It's neccesary to act soon = Action is required soon.
As you can see, this is very different to the others:
- 'Necesitar' is just the verb:
Yo necesito I need
- 'Necesitado' can refer to the participle (I have needed); but it's more usually used as needy.
Esto es para los más necesitados This is for the neediest