To welcome someone in Arabic you can say مرحبا  (marhaban ) and usually you can say سلام (salam ) when mean peace 

when it's morning you can say صباح الخير (sabah alkheir ) which means Good morning 

when someone ask كيف حالك (keif halek ) which it means how are you you can answer جيد (gayed ) 

or بخير (bekheir )  

if you don't understand what someone says you can say أنا لا أفهم ( ana la afhem ) which mean i don't understand 

if he was talking in a fast way that you don't get words properly you can say تكلم ببطىء من فضلك ((takallam bibuṭ' min fadlak) which mean can you talk slowely 

when someone asks you about your hobbies he would say ما هي هواياتك (ma hia hwayatek ) which mean what's your hobbies 

there is some popular hobbies which can be used commenly 

السباحة (alsebaha ) =swimming 

التنس (tennis ) it's almost the same 

القراءة (alqera' ) reading 

السفر travelling (alsafar )

and if you would end the conversation you can say 

اراك لاحقا (arak la7qan ) see you soon 

سلام salam (peace )