The first say at dibden is all about maths and pilapt.the maths is basic arithmetic such as 25x12 and then moves onto addition and subtraction . There are currency conversions e.g.how much is $70 in £s if £1 = €1.20 €1=$1.30
Then you move onto pilapt which is the the pilot aptitude test . Maybe practice using a joystick but I wouldn't advise paying for tests online . You either have it or you don't . I didn't bother practicing this section and I passed .
And yes I'd recommend wearing a suit. The day I went there were 24 people in 3 different groups and everyone was wearing a suit including myself so it's a must.
Any other questions let me know !