The are many ways to show appreciations when you get help or gift from others.
First,you can say "Thank you" explicitly!You can say "Thanks","Thank you","Thank you so much","Really appreicated" etc.And this is the most common way.
Second,you can don't need to say "Thanks" or "appreciated" explicitly,for example,When you get a gift,you can open it and say "oh,that's so nice,I really like it" to show your appreiciation.People will feel that their efforts to buy gift for you is appreciated.This way is very common in foreigners.It is not very common in Chinese because Chinese people don't ususaly open the gift in front of the giver.
Third,you can do some thing in return later on to show appreciations.By doing that,you are building even better relationship with others!
So,please do show appreciations,it is easy,and it makes both you and your friends happy!