Yes, you can apply to be bar staff before you are 18. Depending on your skills and attitude they may even employ you to help behind the scenes, as a waiter, or offer you training in advance of your birthday. Of course the employer will need to work within the law so you will not be able to sell alcohol until you are over 18, and there are other concerns such as health and safety and late working hours that they will need to take into consideration, but if you convey the right working attitude from day one you will be on a better footing to find the job that you want.