My brother has a couple of friends who are policemen and they love the job.
At school it was what I aspired to do as a career, but the civil service was an easier option.
At 18 and a bit you are still young, if you were my son or daughter I would advise you to wait until you were about 21 before joining. This would give you a chance to experience life and would give you a deeper understanding of the people that you are to be dealing with.
Whatever you decide - good luck.