One common piece of writing advice I often see is to write what you know. In fact, just a few days ago, I found a fellow blogger that had asked others about the best piece of writing advice they had ever received, and one person immediately responded with, "Write what you know!" I held my tongue, but my fists involuntarily clenched, and my lips formed a tight, white line. So after having some time to cool off about it, I've given it some thought and would like to approach this idea with a few (hopefully insightful) thoughts of my own.
There is actually some merit to the idea of starting out by writing things you know. All writers start out with topics, styles, and characters that they are familiar with. There's nothing wrong with that at all. In fact, it makes for a great foundation to your writing skills. But once you've learned the basics and how to express yourself in a way that is directly relatable to your own experiences, it's time to stick a toe into the icy waters of the unknown. Now, that's not to say that you can't fall back on the stuff that you're most comfortable with from time to time. Any writing is still practice, and there is always room for improvement, even with the stuff you're good at. But if you don't take that plunge and venture into the unknown, attempting to write something that you're unfamiliar with, you'll never learn or grow as a writer.
When the time has come for you to start that new adventure, there are a few things you should keep in mind so that you don't drown in your attempt. First, be observant and ask yourself questions about the character/situation you hope to portray in your writing. Watch others around you, and perhaps even interview those with the experiences. If it's something you can't learn first-hand, hearing someone else's account of it is often the next best thing. For some helpful tips about interviewing someone for a book or other piece of writing, check out this blog: http://fuckyeahcharacterdevelopment.tumblr.com/post/40749514427/how-to-interview-people
Second thing to keep in mind: do your homework. Research. No, it's not always exciting, but it's necessary for making your writing the best it can be. If you're unfamiliar with a subject matter, what better way to get acquainted with it than to read about it? When you really know what you're talking about in your writing, it shows. Your piece will become not only more believable, but more enjoyable to read as well. Make sure you conduct your research thoroughly enough that you can approach a topic from multiple angles too. That will buy you even more credibility and respect from your readers.
Lastly, stick with reliable sources. The Internet is a bountiful source of information, but being such (and particularly being a place where anyone can publish anything), it's best to check credibility of your sources before actually using them. Stick with sites you know are legit, or sites that at least use citations, particularly with medical conditions. Check more than one site too. If you come up with two completely different sets of "facts" about something, you know something isn't right. And of course, the library is always a fantastic place to find information. It's a bit old school, but you can be sure that the information reliable. Just make sure it's up-to-date; that holds true no matter where the source.
Once you have all the information you need (and citations if necessary), put it to good use. Practice writing the scene, essay, paragraph, or what have you with your new-found knowledge, then take a step back and proof read your work. Do the words reflect your intended thoughts? Do they match up with your research? Don't worry if this isn't the case or if something about it isn't quite right yet. That's what practice is for. Take a break from it and try again later, or keep at it for as long as you like. Whichever method works best for you.
In the end, you'll be rewarded for your perseverance in tackling a new subject matter. Your writing will likely improve (as well as your knowledge and/or experience with the subject). Remember, one of the most important things to keep in mind as a writer is to try and try again. If there is no movement, no growth, in your journey of being a writer, your writing itself will grow stale and uninteresting, ultimately leading to failure. This is one of the biggest reasons why I disagree with the advice to solely stick with what you know. Being bold and taking chances can be very beneficial to a writer; you just have to go about it in the smartest way possible.
"Whoever ceases to be a student has never been a student." - George Iles