I've not really submitted photos for publication, but have worked as a journalist. What Wayne said is important... get published somewhere, anywhere. It might even be a good idea to volunteer your services for small publications or even charities' newsletters; build a portfolio.
Pay attention to BMbikerider's comment about being turned down - expect it and don't be dismayed. Rejection is part of the deal - expect a lot of it. Rejection can be for many reasons, and not always the quality of your work. Perhaps it doesn't fit what they are looking for, or it may be very good but they have better. It could even be nothing more than lack of space.
Back to Wayne's comments, you want to educate yourself on the process - that includes publishing rights. Are you willing to sell the copyright to the magazine? Are you comfortable with licensing to the periodical exclusive rights to your images for a predefined period of time? No matter what, insist on a photo-credit.
Another thing you may consider is starting very small. See if you can find any syndicates that may be interested in your work. If published this way, you may not be paid well and may not even be published in a manner you would like - but regardless, if you are published, then you are building portfolio material.