It varies. Normally you'll sit a set of tests (two or three depending on the company), such as -
Group Bourdon ("dots test", marking groups of four dots off a matrix to test concentration)
SCAAT (same as group bourdon but with alternating shapes rather than dots)
Customer Service Verbal Test (comprehension test, finding answers to multiple choice questions in a passage of text)
Rules Aptitude Aquisition Test (same as above but with a rules theme rather than customer service)
Ticket Selling and Checking Test (basic maths working out ticket prices and change, plus testing concentration and attention checking tickets and identifying whether they're valid, and if not, why.)
Numerical Safety Test (just a timed maths test, simple maths, plus working out fractions and percentages etc)
Officially you can only fail these tests twice before being excluded, but some companies are stricter than others with regards to this.
Then you'll have often have company specific tests such as a group discussion, PA announcement tests, role plays or similar.
You'll generally then have an interview with a manager, which is a mix of competency based questions, and things about your knowledge of the company and the role.