You can say أَهْلًا (pronounced : Ahlan) or مَرْحَبًا (pronounced : Marhaban). There is also السَّلامُ عَلَيْكُم (pronounced : As-slamu Alaykum), and to reply to this greeting, you say وَ عَلَيْكُم السَّلام (pronounced : wa Alaykom As-salam)