Hello ,
I think :
Verb to think in the present tense , indicating that the action of the verb ( here think) is taking place in the present . It could indicate a habit or a fact as well .
I think it is a good idea to call her first .That represents my opinion in the present .
I think English is difficult to learn . That is my opinion in general as a fact.
I always think a lot before taking a decision . It is a habit here .
I am thinking :
It is the present continuous tense ,that is formed by
verb to be + verb in infinitive+ ing :
am + think + ing
It refers to actions taking place at the very moment , now .
Wait , i am thinking about it right now! I am doing that action of thinking right now at this moment .
Hope you got the difference clear :)