I've just started my application. I've been to a careers presentation and have asked to receive the application pack and am now deliberating over the "officer application questionnaire".
You should become conversant with the requirements of the RT, which is the gateway to the recruitment process. Each job has an RT score requirement; Rating entry jobs have a required total score plus specified part scores (for certain sections which relate to the aptitudes needed for the job, such as, say, mechanical comprehension for Engineering Technician jobs); Officer entry jobs have four specified part scores (ie a minimum score required for each of the sections of the test). The bar is set pretty high, so do as much prep as you can before you take the RT. More info here:
Once you have passed the RT with the score you need, you will be able to look at your fitness for the fitness test and start preparing for the Sift and your AIB. At that stage, start getting Navy News from W H Smith each month, to help you prepare.