There isn't a whole lot of difference between the two phrases. They can both be used in the same situations. The only thing is that 'be ready to do' has to be followed by an action, if it has 'do' on the end. If you just write 'be ready to' you can use it with anything eg. 'be ready to go to the shops', 'be ready to run', 'be ready to leave by 6 o'clock'. With 'do' on the end you can only say things like 'be ready to do your homework' or 'be ready to do your chores' because to 'do' something generally only means to complete it, and only tasks or actions can be completed. Without the word 'do' you can use any verb. If you use 'be ready for' you have to change the conjugation of the verb you want to use, in order to make it fit with the rest of the sentence. eg. 'be ready for goING to the shops' (the other way would be 'be ready TO GO to the shops'), or you can use it with an object eg. 'be ready for dinner', 'be ready for the test', 'be ready for bed'. I hope this helps. Unfortunately the difference between the two phrases is a bit subtle so it might be hard to pick up at first.