The main advice I have is that everything is done online these days. Especially for the larger brands in the industry there is nobody to call, you have to start the process online with the companies website. All major cruise brands have a careers tab on their main website. Start there and research what job you might be interested in. Make sure to pay close attention to the hiring requirements, as many positions require a minimum required amount of education or experience.
Depending on the country you are from, you may be redirected to apply through a hiring partner/agency. Large cruise lines employ crew members from over 80 different nations and the only way to handle that many applications is to contract out the initial recruitment to hiring agents in the local country. Most cruise line websites will direct you to the local hiring partner if in fact your country is using one. Be sure to follow up with them and they will carry out your recruitment process from there.
When I was in the hiring process I did as much research as I could about shiplife. I highly encourage anybody considering a life at sea to do the same. There are dozens of videos on YouTube and many blogs and articles from crew members that can paint a picture for you. At the same time, take everything with a grain of salt, some of the videos and articles may be from disgruntled employees, seek out the content that talks about life at sea.
The process can be long, you will need to find some patience as you can go weeks without hearing any updates. Just keep calm and wait. In other cases things might move quickly, especially if there is a significant need for your position.
Have some cash ready. During the recruitment phase if offered a position you may have to secure several foreign Visas, most especially a US C1/D Visa or an EU Schengen Visa. Also, you will have to complete a pre-employment medical exam which varies greatly from one country to the next. For example in the United States and Canada the price can range from $600-$900 and an approved medical is valid for 2 years for most positions, licensed marine officers are required to renew on a more frequent basis.
When you get to the interview phase it is important to make a good impression. You do this by ensuring you research the company, the role and know what it is all about. There is everything you need to know available to you online, google, YouTube and even instagram can help you with your knowledge.