It's the prepositional form of она. You use it... well... with prepositions.
Note that the dative and instrumental forms of она are also either ей or ней depending on... stuff. So don't get confused ;)
"ей" turns to "ней" when used with a preposition. The same is true for all personal pronouns starting with "е". Ему - к нему, её - у неё, etc.
её it is for example a thing "it is her book ( это её книга.)" ей it answers the question whom. I have to help her Я должен помочь ей
I understand that "ничего" is the genitive, with "ничто" being the accusative (same as nominative). But I would have expected accusative here since знать acts directly on "ничто". So now my understanding of "direct object" has to be revised.
Is it because it is a negative sentence? Does the genitive always pop up like that in the negative or is it only for что/ничто? Does it happen for other verbs apart from знать?
This sentence makes me think that it will probably be like:
Я знаю Марию "I know Maria"
Я не знаю Марии "I do not know Maria" (genitive appears because of the negative? I am copying the example sentence here... I thought accusative was used also in the negative).
And the same for люблю I guess... I need the help of an expert now :P
It seems that ничего is now preferred to ничто even for genitive and accusative uses outside of literary work and idiomatic speech.
This thread on the topic (or what I could manage to understand of it, anyways :-P) is pretty interesting: http://rus.stackexchange.com/questions/37548/Ничто-не-интересует-или-ничего-не-интересует
I saw a comment in another thread which stated that the direct object of negated transitive verbs is usually cast in Genitive case. Here is probably an example of that. (According to Katznere's English-Russian-English dictionary, ничего is the genitive form of an indefinite pronoun), so here we have a pronoun direct object of не знаю, a negated transitive verb.
Negation in Russian, from what I've picked up, is a tricky subject when it gets more complex than нет [genitive thing].
would я не знаю ничего о ней work as well? or is it necessary to write ничего first?
Can this usage of ней refer to any feminine noun? So in other words translate in English to "I know nothing about it"
If the context makes it clear you're referring to a feminine object, then yes. Duo translates «я о нём думаю» as "I think about it", although Duo accepts "I think about him".
Is the preposition "o" stressed or unstressed? Is it pronounced as an "o" or as an "a"?
"I do not know about her" or "I do not know anything about her", Where do you get the "anything" from?
In standard English objects of a preposition usually follow the direct object. I wouldn't say what you wrote is necessarily incorrect, but it sounds very odd.
"ничего" is "не́чего" in the dictionary. https://dict.leo.org/russisch-deutsch/%D0%9D%D0%B5%CC%81%D1%87%D0%B5%D0%B3%D0%BE
They're two different words. Нечего means "there is nothing," as in «Нечего сказать.» = "There is nothing to say." Note that the vocal stress also falls on different syllables.