"Somebody needs you."
Translation:Någon behöver dig.
"Någon" means "Someone" because nouns that describe people are (almost) always "En" nouns.
"Något" means "Something".
If it's together with the noun, it's något: 'Finns det något barn här?' (is there a child here). But the word someone is always någon.
As Andrew explained above for the singular case, ni is used when it is the subject of the sentence, such as in 'you are late'. When it is the object, you use er, as in 'we see you in the fog'
Yes, er works if you mean the plural you, the English sentence is ambiguous.
du doesn't work since you is an object here and du is the subject form.
du and ni are subject forms and dig and er are object forms.
You can compare with I vs me in English.
It is now. But it's a much safer bet to stick with "någon" throughout the course.