Following are some words can useful mostly in North of India,as in south there are different languages spoken but most of the people who knows Hindi can understand even in south but very few people,whereas almost everyone in north India.
Namaste= A form of greeting just like hello & goodbye.
Dhanywad or Shukriya = Thank you,Dhanywad is typical Hindi,(Sanskrit origin), language,Shukriya is Urdu,(Like Shukran in Arabic).
Phir milenge = See you later
Aap kaise hain? = How are you?
Yeh kitne ka hai? = How much it costs?
Iska kitna keemat/daam hai = How much is it?.
Regards
Nayak