The two everyday words are शुक्रिया (shukriya) and धन्यवाद (dhanyavaad). In ordinary conversation most people say shukriya; dhanyavaad is more formal and can sound stiff between friends.
There is a third option you will hear constantly: thank you,
in English. In Indian cities this is completely normal and nobody considers it
lazy.
Which to use when
शुक्रिया (shukriya) — a shopkeeper, a driver, a colleague,
a friend. Warm and natural. This is the safe default.
धन्यवाद (dhanyavaad) — formal writing, a speech, a much
older person, an official setting.
बहुत बहुत धन्यवाद — thank you very much, for something
genuinely significant.
The part that surprises people
Within an Indian family, thanking someone for a small everyday kindness can
feel distancing — as though you are treating a family member like a
stranger. A mother who has cooked dinner is generally not thanked verbally. This
is not rudeness; it is a different idea of what closeness sounds like.
With people outside the family, thank normally.
Replying
कोई बात नहीं (koi baat nahin) — no problem, it's nothing.
This is what you will actually hear. The textbook swagat hai means
"welcome" in the sense of greeting an arrival, not "you're welcome".
Practise this with a teacher
Reading an answer is one thing; being corrected while you say it is
another. EkGuru tutors teach one to one from
$6
a lesson, and most offer a trial.