This question comes up in some form or another at least once a week. Which words do we capitalize, and which are lower case? Whether internet/Internet, Iraq war/Iraq War, or some other term, often we are unsure whether to hit the shift key on that first letter.
If you don't know house style rules, here's a safe way to find answers to these questions.
Check the AP Style book.
For the casual kinds of writing usually employed by genre fiction writers, AP style will be a safe fallback position whenever you run into these kinds of questions. Most of us use AP as a sort of baseline for a lot of technical questions like spelling, caps, hyphens, and so on. We may diverge from their rules here and there, but even when we do, it won't hurt you to follow AP. It's an accepted and recognized standard.
What if you don't have an AP style book and can't get to one?
Check Webster's. Not all dictionaries are created alike, and writers have been known to have bitter rows over which is a better standard. Some swear by the Oxford English Dictionary because of its comprehensive etymology. It's great for historical fiction writers. Some prefer American Heritage for its insightful inclusion of regional dialect and jargon.
So why am I advocating Webster's?
Because that's what AP does. When AP doesn't specify a particular spelling, it defaults to Webster's.
I keep multiple dictionaries on hand to check shades of meaning -- comparing definitions across dictionaries can often reveal subtle connotations. But for matters of style, when the Red Sage book is silent, we go to AP and, like AP, to Webster's.
Regardless of which method you choose, be sure to remain consistent. Don't use the OED variant for the first usage of a term and the Webster's for the next. Don't mix your internets and your Internets in the same document.
Which are your favorite dictionaries, and why?