what will you do?

why can't use ” what will you do?” to talk about what you have arranged to do( from my text book)
why can't use instead of will that ”what are you doing this evening?”and ” Alex is getting married next month. (not will get)
when do you use ”will” for future question?

You can't use "what will you do?" to talk about what you have already arranged because "will" is used to talk about a prediction.  "What are you doing this evening?" is used instead because [am/is/are] + ~ing] is used to express a plan.
what will you do after dinner?
Is that wrong?
but sometimes, I've seen this like structures.
I'm not sure.

Again the use of 'will' suggests that a plan has not been made yet.  It sounds a little strange because often people say "What will you do?" to express uncertainty in a negative situation.  For example:
"What will you do if you don't get the job?"

"How will I ever finish this work on time?!"

("What are you going to do" and "How am I ever going to finish" can also be used, but require a more definite answer as they suggest that a plan has been made.)

I would disagree that the answer is 'laziness', as it depends on the context and what you are trying to express.
Alister is right that 'what will you do...' and 'what are you doing...' can both be used to talk about the future, but they have very different implications.  It is not a case of using one over the other just because it's easier!

'Will' is commonly used to express a promise.

For example:

"Please don't tell anyone my secret..."

"I won't!" (will not)

"I have to go now.

"Ok. I will call you tomorrow." 

If the speaker had said "I'm going to call you tomorrow", it would imply that they had planned it in advance.  Usually we use 'will' in this case if we decide to do something on the spot.  This often occurs when making requests or offers.

For example:

"I'm hungry."

"I'll make you something to eat."

"I don't understand my homework."

"I'll help you with it."

We use 'going to' or 'am/are/is + ~ing' to express a plan.

"I'm going to go travelling after university."

"My parents are taking me to London for my birthday."

"My sister's getting married next month."

The reason that you've seen both 'will' and 'going to' used for similar sentences might be that 'will' is easier to teach at first.  But once you get used to how each phrase is used in different situations, I think you'll understand it more easily! 

I hope that helps :)

The answer is laziness.  Some phrases work for the present and the future.  For example:

"What are you doing now?"

"What are you doing tonight?"

"What are you doing tomorrow?"

"What are you doing next year?"

Other phrases only work for the future:

"What will you do tonight?"

"What will you do tomorrow?"

"What will you do next year?"

The phrase "what will you do" cannot be used to talk about the present.  Therefore, the phrase "what are you doing" is more flexible.  We are lazy.  If we can pick a phrase which we can use everywhere, that is the phrase we will use most often.

Now consider the phrase "is getting".  This phrase can be used for the present and for the future:

"Alex is getting married now."

"Alex is getting married tonight."

"Alex is getting married tomorrow."

"Alex is getting married next month."

However, the phrase "will get" can only be used for the future:

"Alex will get married tonight."

"Alex will get married tomorrow."

"Alex will get married next month."

The phrase "will get" cannot be used to talk about the present.  Therefore, the phrase "is getting" is more flexible.  We are lazy.  We will say "is getting" more often, because it is a more flexible phrase.

I hope I have made sense.

what will you do?

why can't we use ”what will you do?” to talk about what you have arranged to do ( from according to my text book)  I think the "will you do" is implied conditional - but planned is not conditional, that I guess is what the text is hinting at.  But the rule is not correct.  

After dinner we will definitely go to the movies!

why can't we use ”what are you doing this evening?” instead of "will you do" that 

Well you can, but I think the first one implies "what are you planning on doing..." since clearly you cannot be doing something that occurs in the future.

and how about ”Alex is getting married next month. (instead of not "will get")

again I suggest 'is getting married' is short for 'is planning to get married' for the same reason

when do you use ”will” for future question?

what will you do after dinner? = what have you decided to do this evening

what are you doing (planning to do) this evening
Is that wrong? it is correct!
but sometimes, I've seen such sentences occasionally this like structures.
I'm not clear sure.