Basically, I think I am a good person and I am very friendly to others, b. But because I am always shy, I usually avoid helping others.

That sentence is fine spoken, but as written it becomes too complicated with too many clauses. You can also write this without "but".

Next year, I want to join more public welfarcommunity service activities.

In US, people don't say "public welfare activities". Public is used more in politics and welfare is used more in economics or in terms of governmental benefits.
Community Service is the common term in the US at least.

Also, no comma after "Next year"

Thus, I am good for our society and I also hopeis way I can be good for our community and, hopefully, I can make myself happyier, too.

I changed a lot here.
Thus is too formal and really didn't fit here.
Society is too large - it's almost at the national or higher level. Community works better here. If you want "society" then I'd delete "our".
Last thing to comment on: ""make myself happy" = I am sad now and this will make me happy. "make myself happier" = this will contribute to my happiness.

The last plan has nothing to do with foreign languages.

You cant really say the last plan if there is no context

I want to actively help others more often.

Thus, I am good for ouwould be helpful for society, and I also hope I can make myself happyat the same time I would be helping myself or I would feel happy knowing I have helped others.

I think you need to use Would as you are discussing the future - I think the word good is overused, try to think of what exactly is good? helpful? may be better