Before booking the exam
First, remember that the test costs money so make sure you actually need to do it – for example, some schools prefer TOEFL to IELTS. If it for a university or visa application check whether they are asking specifically for the General IELTS or Academic IELTS and for what score you should be aiming.
Remember the exam tests 4 key skills: speaking, reading, listening, and writing. Make sure you understand the format.
It is also a good idea to do some practice/study before booking the test. IELTS does not run IELTS training courses itself, but there is a major market in schools and tutors who can help students prepare.
Cambridge Press publishes books of past IELTS papers. These books include papers and example answers; they can be a good way to assess your own level as well as practice.
Booking the exam
The British Council has a page on which students can find the dates and closest exam centres:
http://takeielts.britishcouncil.org
Payment will be due upon booking the exam.
Exam Days
The IELTS exam often happens over two days: one day during which examiners mark speaking, and another for the writing, reading and listening sections.
Turn up in good time. Latecomers may be refused entry.
Bring the same ID that you entered on your application form.
Also bring two identical (and recent) passport photos.
Read and listen to instructions carefully. Losing points for not doing what is desired is a terrible waste.
No cheating
This is an international test that is very important to many students and organizations. As a result cheating is taken very seriously.
More information
The British Council have their own advice page:
http://takeielts.britishcouncil.org/prepare-your-test/test-day-advice