I am starting to understand the Arabic language. As my first arabic book I want to buy the tefseer from imam sadi. Which arabic books who you recommend for a beginner. And do you know a trusted website where i can buy arabic/islamic books?

In regards to what books to buy, I would definitely encourage you to buy short books, like booklets. Start with the fundamental texts from the Mutun: 1. The Four Rules 2. The Three Fundamental Principles If you see a brief, short text by Imam Ibn Baz or Ibn Uthaymeen, buy those. They have so many beautiful writings in all aspects of the religion that are very beneficial, especially in regards to: • Salah • Following the Sunnah • Hijab Get some of these booklets so you can practice reading and understanding them, Insha'Allah.