You 'join' easyjet on your induction day (the day before you go down to CTC normally), but your details are not passed to the payroll department until you have finished your pre-base-training simulator session. This is your last day in the sim and occurs after you have done your LST. Once you have successfully done all that you will be included in the payroll system from that day.
However! I believe the cut-off date is the 10th of the month or something, so if you do your pre-base-training detail on the 15th of January, for example, you will start earning money from that date but will not actually receive a paycheck until the end of February.........(When you do it will be for all of Feb and half of Jan.)
So it's a long time to wait.