There were many contributing factors for Arabic. I studied Arabic in primary school, which means I have some know-how of Arabic grammar, tenses and translation. Secondly, my elder brother had opted Arabic in CSS. I had his guidance and notes too.
"Taareekh-e-Arbi Adab" by Abdul Haleem Nadvi is a good book.

As far as writing in Arabic is concerned, the knowledge of Arabic sentence structure and "Gardaan" are important. Reading Quran helps A LOT. It increases your vocabulary.