Hi
amoghsharmaI did IELTS and TOEFL before. they are no deal compared to GMAT. I bet you will enjoy it.
TOEFL is completely different from GMAT. TOEFL test your ability to LISTEN, WRITE, SPEAK and READ in common life rather than harsh academia as that of GMAT.
since your are an English speaker, you won't need more than 1 month. All you have to do is to see some videos about it, then practice some sample tests to be familiar with the real one.
Mostly, you will find the listening, reading and speaking parts easy for you and will only need little practice.
The one that you have to stress on is the writing. I believe SC of GMAT taught you a lot and built for you a better grammar sense. You have to practice on the writing tasks and the vocabulary needed to perfect them.
If you are taking ibt TOEFL, try to improve your typing speed because it will not be writing with a pen, but with a keyboard.
Also take care of the spelling mistakes because this is a part that is not covered by GMAT.