To register for an IELTS test, you can download an application form, fill it in, then walk into the center nearest to you, and the staff at the center will help you register.
The application form can be downloaded from here:
You can also register online at this official web page:
There is currently NO restriction on the number of times that you can retake the IELTS. Also, there is currently NO requirement to maintain any gap between tests.