This can't be guaranteed. A flu jab protects you against the virus by making antibodies that fight it. While this usually works, you can't be sure that the antibodies will always succeed in fighting off the virus. They also become less effective over time as the virus learns to overcome them, which is why a new flu vaccine has to be developed every year to create new antibodies to combat the latest form of the virus.
Nevertheless, a vaccine does reduce your chances of getting flu as much as anything can, and should make the symptoms much less severe if you do get it. It's not safe for everyone - some people have an allergic reaction, for example - but most people would get some benefit from it.
However, if you took the vaccine every year it wouldn't necessarily be as effective. Also, as the supply is not unlimited it should really be given to the people who are most at risk from flu, such as the elderly, pregnant women etc. For most of us, flu is a very nasty illness but we fully recover from it with no lasting effects, and if your general health is good and you don't belong to an at-risk group you are normally just as well off staying in bed, keeping warm and drinking fluids until it passes.