Commercial photography requires taking pictures that people want and finding ways to sell them. You can do this on a freelance basis where you shoot a lot of images and then sell them or you can shoot by assignment where you shoot specific subjects. No matter where you want to get started, you're going to need a strong commercial photography portfolio.
Think About the Market
Understanding what the art buyer wants and needs is critical in building your portfolio. If you're not sure you want to focus on a specific niche, it is a good idea to take a wide variety of marketable photographs that will demonstrate your flexibility. This will also increase the chances of a buyer seeing something they like.
Army Photography Contest - 2007 - FMWRC - Arts and Crafts - A Plumpish Proportion by familymwr on flickr (licensed CC-BY Photo By: SSG Robert Stewart)
Don't forget that any pictures with people in them will need to have signed releases letting you sell the images so it may be easier to focus on non-people pictures when you first start out. (If you do want to photograph people, a quick web search for 'Model release' turns up plenty of example forms you can use).
There are two reasons that the Internet is your most powerful ally in building a commercial photography portfolio. The first is that it allows you to search through the portfolios of thousands of working commercial photographers and see what sorts of images sell the best.
It also provides you with a way to get your portfolio distributed to as many people as possible. Register a personal domain name and upload your portfolio, then send a link to it with any correspondence you have with publications or buyers.
Create Your Own Assignments
When you first start, you're not going to have assignments right away, so practice with "fake" assignments to demonstrate your skills. For example if you want to do advertising images, create a couple of made up ad campaigns and take some photos that would go with them. Then put them on your portfolio explaining how they interact with the campaign and why you selected them. Art directors love a photographer that can think "in theme".
Pick Your Niche
One of the best ways in which to profit from photography is by becoming the best photographer at a certain kind of photographs. There are photographers who focus just one subject matter like food, celebrities, food, animals, bikinis and many others.
For example, www.weshootbottles.com specializes in bottle photography. Find a niche that you like, and buy a couple of publications that you'd like to shoot for and build your portfolio to fit that potential client.
When you're first getting started it can be difficult to get on with a major publication so start off focusing locally. Find out if there are any local newspapers, magazines, websites, or communities that need photo work done and volunteer your services.
You may not be rolling in the dough right at the start, but you will be building a powerful list of former clients and a healthy bit of experience, both of which will enhance your chances of landing a bigger and better assignment in the future.
Commercial photography can be tricky market to get into but with a positive mindset and a eye for spotting local needs it may be easier than you first thought. Challenge yourself to expand on your skills with every shoot and to reach out to businesses and colleagues and you'll have a powerful portfolio in no time.