You may find that part of the assessment process requires you to take aptitude and / or psychometric tests.
There are many different types of test, but these are amongst the most common:
You cannot really prepare for such tests, other than by making yourself aware of the format of each type of test. There are lots of books and websites that can help you and provide example tests for you to try (e.g. www.psychometrics.co.uk and www.shl.com).
The most important thing with the personality test is to be yourself and to be as honest as possible. The test is not designed to trick you, so don't try to be clever and second-guess the answers because you will be asked the same thing in several different ways. Remember that it's for your good as much as for the benefit of the company - do you really want to work somewhere where your personality doesn't fit into the business culture?
When you take the tests try to stay as relaxed as possible. Listen to or read the instructions very carefully and don't be afraid of telling your assessor if you do not understand something. Most tests are timed, so work quickly and accurately without wasting time on questions or statements you don't understand - remember that it is quite common not to complete the whole test.
Finally, it is just as important for you to be given feedback after the tests as it is for the company, because this often provides useful information for you, whether or not you get the job. So, if feedback isn't offered, ask for it.
Good luck!