ST69.
All praise to you for coming on here to ask advice but if you can't raise your sights above Media Studies than you will be in for one hell of a shock when it comes to basic, and professional, training.
I cannot emphasise the mental maths angle as well. You will be amazed at how difficult it is to add two numbers together when you are in a dynamic and stressful environment.
So, best to start training your mind early. Pick a demanding subject and practice gymnastics with your brain. Been out of the trg loop for a while so i will leave it to others to give you pointers in that direction.
Having said that, practice compass bearings. By that I mean, look at a heading and then work out the reciprical heading, and then 90 degrees off that, then the reciprical off that and so on. Do the same, but counting through North.
Will stand back for others with better ideas.