I am not sure if I am understanding the issue correctly. This is my understanding....you want to create a sort of form where a user can input but cannot alter anything including design, formula and so. 

If this is case, you need to two things

1. first unlock cells where user will be able to input the number. 

Select all input cells -> Right click -> format cells -> protect tab -> uncleck  Locked.


2. You protect sheet using review tab -> protect sheet fuction.

Assign password for the protection. You can passport protect it so that if you need to change anything you can unprotect it...make changes..and protect the sheet again.

This will solve the problem.

This is not like password protecting the whole workbook which ask users to input password as they try to open workbook. In the case explained above, user can open the workbook but will not be able to change. But there will be able to input numbers where you have unlocked in step 1.

Hope this helps.

This is very basic functionality. You can easily find turorial about it.
You just protect the sheet and add password to it. Don't just read 'protect the sheet'. You will have multiple options for the targeted cells.