I cook for myself so have some experience there, I also managed to get some experience through work years ago too. It helped when it came to answering things about food hygiene, however, you can look that sort of thing up online. They will train you in everything you need to know and how they work so don't worry too much about that.
They also put you through the training for secondary job roles as well so its definitely worth looking at those and the training involved. They are mandatory MCA certificates required to work at sea as is the ENG1 medical.
There are plenty of people that apply for the role as a career change with no experience at all so they expect people will apply with little or no experience to apply. I guess if you can get some relevant experience, it will show people that you have the interest and desire for the role and will put you ahead of other people your age who have not made any effort to research things or get experience. You could always see what the recruiting people say.
What I would say is this, working at sea as a chef is going be totally different to working on land. Think about the working environment. There are waves that make the ship unstable, weather conditions too as you could be going through storm force conditions which affect things as well, not forgetting the fact the ship will change course and speed as well. Think about the fact that if you're in the middle of the Atlantic/Pacific/Indian Ocean, you can't quickly order stuff if you run out of stores and also there's the issue of what to do about rubbish so bear that in mind too. Also think about what it's like living at sea too as you can't escape if you're having a bad day.
Do not let inexperience deter you in any way. As long as you can show that you are keen and interested, that's important too. Think about why you want the RFA Apprentice Chef position as opposed to doing it somewhere else. Try to really explore the RFA website, the job role, other posts here on the forum, other websites etc and think about how/why the RFA is the place to be.
Hope it helps.