They are different letters. "N" is like the English "N". The letter "ñ" sounds like "en-yay" by itself, but when used in a word it is very close to "ni". For example, "niño" sounds like "ninio".
For a beginner I would recommend pronouncing ñ as an 'ni' sound.
Once you hear the ñ sound a few times yourself you'll learn that its a much harsher noise than just 'ni' but it's a step in the right direction I would say.
Both are correct. In English, we generally don't apply masculinity/femininity. In Spanish, niña is the feminine version of child. So, saying "Ella es una niña." means "She is a child." with female being implied.
"él" requires an accent when it means "he", and no accent when it means "the". The accent is only there to differentiate the two meanings. "ella" has no other word that sounds like it, so it has no accent in this case.