These come up in almost every conversation. Learn them whole:

French	English	When
Ce n'est pas grave. / Pas grave.	Never mind. / No big deal.	after an apology or a small mishap
Pas de souci.	No worries.	replying to thanks or a request
Ça m'est égal.	I don't mind.	when the choice makes no difference to you
Ça marche !	Sounds good! / Deal!	accepting a plan
Ça te dit ?	Do you fancy it?	suggesting something
Ce n'est pas vrai !	You're kidding! / No way!	surprise, good or bad
Bon courage !	Hang in there!	to someone facing work or an ordeal
À plus !	See you!	casual goodbye — A+ in messages

Ça m'est égal is neutral, but said flatly it can sound like I couldn't care less. When someone offers you a choice, comme tu veux — whatever you like — sounds warmer.