DON'T wear a tie.!!!!!
Best to look a little more casual, they like that down in the nerve centre. Makes the Orange Injection much more easy to administer. Expect a jolly day with a tech questionnaire, group exercise (could be anything, but usually includes bits of string and plastc tubing).....another mental exercise with groups of cards with missing data that you have to find...timed to 30 mins I think. Dont worry about finishing that, its more quality than quantity.
Erm.......oh yes, phsycometric (sic) test. 400 questions about loving rabbits etc.
Interview with senior training captain and HR at the same time (well mine was).
Get through that lot and its a pat on the back and "see you tomorrow for the sim".......
Sim for me was 737-300. Manual flying, no F/Ds. SID from Luton, bit of speeding up, slowing down (watch for that pitch thrust couple on the baby) steep turns etc.
Enjoy the day/days and you'll be fine. Best thing to wear is a smile...
Good Luck, see you on the line.