Most people follow the Aptitude tests the next day, at the Choosing Ceremony, but the final choice is up to them. Each child takes this test at 16, and the next day the Choosing Ceremony will take place.
Just A Simulation; Not Real
They will have an assorted series of tests which you must go through to determine what characteristics you have, from there it will decide what faction you should be in; but the choice is really up to you. Through out the Simulation Tris has to remind herself that it is just a simulation and that it is not real; she won't really die. However, I won't spoil the tests for you; you'll just have to find out for yourself.