Takes me back a few years to when I had a similar dream.
With hindsight I would say preparation for the interview (reading up on the news, specifically defence topics) is important.
Keep fairly fit but there is nothing you can do about the medical.
Have a look at some aptitude tests but don't get hung up on them. I am convinced that reacting well under pressure can help - I was extremely lucky as I made a big mistake with one of the assessments but was able to come up with a plan on the spot that worked.
When I went through the assessment I think the leadership exercises were crucial - you should get some ideas through contacts in ATC.
Censored and frowned upon it may have been but the phrase from the dreaded 'Fighter Pilot' series still holds true - "Nobody wants a ***ker on their wing"!
I am now retired so probably out of date and over the hill! Good luck.