make ready for有一种防范的感觉,意思就是,不希望FOR的那个东西发生．．
You had better make ready for danger.
get ready for就是准备的意思～
You should get ready for your exam.
区别不大,你如果说,get ready for danger．老外也听得懂,不过是习惯上的搭配和意味的问题～