First and foremost,when applying for all the differant trades that airlines offer,tick the "Avionics" box.
Secondly,when filling out an application form,if they ask you to choose your trade of choice from your most desired to least desired,have a "Good" think about what trade you want to do,then,tick "ONLY" the Avionics box and no others.The reason I say this is ,firstly it will tell the employer that you know exactly what you want to do and that you dont want to accept second best.Admittedly this might be a gamble,but this gamble will have to be weighed up personally against your schooling results.Going back a little,I dont really want to put you off choosing an apprenticeship in the Aircraft "Mechanical" side,as I still think this would be a fullfilling carreer path,but being an avionics Lame myself,in hindsight I am glad I chose avionics as their are a lot of mechanical Lames around "not all", who are bitter for not choosing avionics.Suspect I would probably be the same.
Post schooling advice,approach a TAFE college about possible pre apprenticeship courses if any.If you miss out the first time,dont give up until the airline is sick of hearing your name and they employ you to shut you up.HA HA.
Just quietly,dont know if this applies to you or not,but you will be DRUG tested for entry to an airline so keep this in mind
Lastly,dont listen to all the really negative comments about ****ty pay etc, as a lot of these stem from people in GA, which is light aircraft. I myself earn around $100k a year,but this is nothing compared to what you can earn as some of my collegues earn closer to $200k a year,for a very interesting and challenging career..
Best of luck.