I learned my great uncle joined at 17 in '44 and was out in 18 months when the war was over in '45. In that time he became a fireman 2nd class. My husband thinks he got there too fast compared to today's standards. But what would he have been doing?
A fireman would work on the ship's engines and their auxiliary machinery, ensuring they ran correctly and performing maintenance. In the Royal Navy, they were called stokers, a hangover from the days of WW1 and earlier, when they had to stoke coal-burning furnaces. By WW2, most navies had transitioned to oil-burning machinery, and firemen or stokers ensured that this ran as it should. The RN expected that, under the supervision of an engineering officer or petty officer, a stoker "tends coal or oil fired boilers, keeps watch on main and auxiliary machinery. Reads simple recording instruments, inspects self-lubricated bearings, oils open type engines, runs fire and bilge pumps". In the RN, stokers were recruited directly, and entered the service as stoker second class. After a year of service, they were eligible for their first class rate, though this could be brought forward if they were exceptional. Two more years of service, a watchstanding qualification and passage of an exam made them eligible to become a leading stoker, which also came with three months of further training in mechanical skills. A stoker second class would be taking orders, not giving them. They would work on the ship's boiler, the engines and turbines it fed, or on auxiliary machinery like desalination plants. In port, they were responsible for cleaning and maintaining the boilers or other machinery.
Just to clarify, which navy are we talking about here? /u/thefourthmaninaboat gave an excellent answer concerning the Royal Navy, but not all navies work exactly the same way.