The first step on the road to become a documentary photographer is to learn as much as possible about photography. While digital cameras make taking photos much easier, documentary photography goes well beyond simple point-and-shoot picture taking; you will need to learn how to use the camera properly, as well as understand different lighting options such as flashes and strobes, light modifiers, and other techniques for properly metering a shot. Taking photography classes or apprenticing with a professional photographer is a good first step if you want to become a documentary photographer.
Once you have developed some photography skills, you will need to start building a portfolio that will help you become a documentary photographer. Take as many photos as possible, experimenting with different techniques and subject matter. Design and implement your own photography projects that will help you become a documentary photographer by building a body of work that showcases your skills in the documentary realm. Your portfolio should showcase only your best work, and it should be constantly evolving. Taking photos of events and people should be a paramount concern for any job or project you take on, though any photo assignments are good for developing the portfolio you will need.
You will need very good people skills if you want to become a documentary photographer. It is essential to be able to engage with people, hold conversations, and capture moments as they happen. This means being able to read events before they happen or as they are happening, which requires a bit of experience, a lot of savvy, and a significant amount of courage. Try to push beyond your comfort zone to get the best shots and to develop your skills even further. Don't be afraid to ask people if you can take their photos.
Protect yourself. Draw up model releases that you can carry with you and give to people you take photos of. This will ensure you are protected in the event of legal disputes, and it allows you to use the photos for personal or commercial use. You may choose to work on your own, or you may be able to get hired as a documentary photographer for a magazine or other publication; either way, make sure you are protecting yourself and your work throughout every project you undertake. Don't be afraid to take assignments that will push you beyond your skill level or comfort zone; these jobs are often the most valuable for developing new skills and improving your portfolio.