For enlisted Marines, visit your friendly local recruiter. If you meet all requirements, and the recruiter deems you a solid candidate for becoming a Marine, you will take a military entrance exam. The "ASVAB" which will gauge your strengths and weaknesses, and crank out a bunch of scores. Of these scores, the most important are your Overall (AFQT) and General Technical (GT) scores, which will often determine the types of jobs you're qualified for.
Some jobs (like Public Affairs/Public Relations) will require a brief interview with a regional director. He/she is an active-duty Marine officer, usually a Captain/Major. The interview is extremely "soft-ball" and is just to ensure that you have potential as a PA specialist. Frankly, if you made it to this point, the Marine Corps does not want to turn you away. They've essentially made up their mind that they want you, just don't completely freeze up or say anything ridiculous during the 5-minute phone interview.
If you sign the contract (usually 4-5 years) you will then set a date for going to Boot Camp (3 months). Following boot camp you receive 10 days off, then you go to 3-4 weeks of Marine Combat Training (MCT) before going to your 3-7 month Military Occupational Specialty (MOS) school at Fort Meade, MD, which will teach you the basics of Marine Corps public affairs and journalism (print/video).
As long as you continue to pass all physical requirements and requirements in your general Marine Corps training, and you keep solid grades throughout school, you will be working as a combat correspondent (military journalist) for your first several years. This will evolve into media relations and analysis, product marketing, community outreach and event planning, speech writing, social media management, as well as training and personnel development.