Some companies may encourage you to apply online while others will need you to apply to them in writing. You will need to decide whether you wish to train as a Navigation(Deck) Officer or Engineer Officer. There are also some places for Electro-Technical Officers. Look at the list of companies for contact details - they will advise you on their requirements. If they do not have online applications they will usually send you an information pack containing their application form.
You do not need to wait for your academic results before applying, as recruitment may be possible anything up to one year in advance of beginning the course and there are multiple entry dates, usually September and January and possibly March/April.
If you meet the company's initial requirements you will be invited for an interview.