We use "going to" when something has been planned in advance and is now a firm plan. Example: "I'm going to visit my grandmother on Sunday." We use "will" when we have just made the decision. Example: "Are you going to McDonalds for lunch? I'll come with you." Sometimes we use "will" to express a very strong decision. Example: "I will never text while driving again! It's too dangerous!"

be going to do - it means you not sure that you do anything, may be yes may be no, will do- means you sure, you do it exactly

thank you for your help :)

They have very similar meanings. The first one "I'm going to take a bus to visit my friend" makes it sound like you are in the process/action of going right now. "I'll take a bus to visit my friend" tells someone what you plan on doing, but you are not necessarily doing it right now. The first sentence could also be said without doing it right now, but if you were leaving to take a bus right now, you probably wouldn't use the second sentence. Hope that helps!