For Shell, I will advise you log on to shell's career page and fill in the necessary details.
If you are experienced, I assure you that your CV will be considered whenever there is an opening in your field.
Often times, placing an advert for a single position on the national dailies or other media is not cost effective.
For trainee level, just hang on till from now till May if they are still continuing with the SITP1 stuff.
Well, my candid advice for you is to start from anywhere you can get the necessary experience to move on in life. You may not get into the "Big four" now, but later.
However, you will only succeed by the grace of God. So, hold on to Him with all diligence.
Jesus loves you.