hacer falta vs necesitar
Both express need. Necesitar puts you as the subject. Hacer falta works like gustar — the thing needed is the subject.
necesitar
To need — the person who needs is the subject.
Necesito ayuda.
I need help.
Direct and universal.
Necesitas descansar.
You need to rest.
Necesitar + infinitive.
¿Necesitas algo?
Do you need anything?
A polite offer to help.
hacer falta
To be needed or lacking. The needed thing is the subject, the person is the indirect object.
Me hace falta ayuda.
I need help.
Same meaning as 'necesito ayuda' but with the gustar structure.
Hacen falta dos personas más.
Two more people are needed.
Plural noun gives plural verb: hacen falta.
No hace falta que vengas.
You don't need to come.
No hace falta que + subjunctive. Very useful way to say 'no need'.
Tricky bit
'No me hace falta' = I don't need it. 'Me hace falta' = I need it. Learners add 'yo' as the subject and get 'yo hago falta', which means 'I'm needed'.