An 18 year old entering the workplace for possibly the first time will either be looking for a career-starting position in a chosen field or profession, or looking to gain as much experience as possible to get the best start for himself. This attitude will determine the places and ways an 18 year old will look for work.
Internship or Volunteering
If you have a particular interest in a certain career field, an internship or volunteering is a great method in putting your foot in the door. Even though it does come with a negative side of receiving minimal to zero pay, the positives are that you will gain invaluable experience and create a vast network base. The payoffs might not be immediate, but in the long term it will allow you to basically choose who you want to work for or, if you are really ambitious, start your own business as the skills you will gain as a volunteer or intern -- for example, working knowledge of an industry or organizational skills -- may prove helpful in designing and executing a business plan.
Many 18 year olds are recent ex-high school students. Accordingly, the lessons they learned in school are still fresh in their minds. There are plenty of parents of students struggling to get by with some compulsory subjects at high school who will happily pay for good tutoring if it proves to help their child. An hourly rate can be negotiated and the tutor can make very good money.
If you are interested in ultimately becoming a highly-experienced professional, you could chase a job in your chosen field by looking to enter the profession at entry level with the sole, driven determination of bettering yourself and raising up through the ranks via promotion. This could be undertaken whilst completing studies.
If you have great communication skills or have the ability to persuade people, a sales job can be the right career for you. There is always a market for people who can sell a product or service. It is a fast-paced environment and the earnings predominantly depend entirely on how you interact with potential customers. There is also the safer option of base salary sales positions, which is a steadier position; however the earning potential is also less.