Approach the relevant editor with a clearly thought out idea and treatment. You need to demonstrate that you can write informatively and interestingly and you also need to have considered how to illustrate the article.
The editors tend to get quite a bit of stuff on spec and dont always get time to read it all. What is really important to them is that you can deliver quality, on time, to a clear brief.
You should also expect to get paid if your articles are good enough. Certainly the mag I am involved in does not engage in vanity publishing and will only publish material which creates interest for the reader and therefore is worth paying for.
So in summary:
Find an interesting article
Write it well and to the spec agreed with the ed
Make sure you have good pics
Get paid
Dont expect to live on the proceeds unless you can keep delivering the goods.