At first I thought that I would be able to deal with capital letters in a couple of paragraphs, but the more I thought about it, the more I realised that they need their own chapter. I will throw in a few extra comments about terms of address, though.
Capitals are used for several different reasons. These include:
- Beginning a sentence
- The pronoun "I"
- Names and titles of people
- Place names
- Names of brands or trademarked objects
- Objects of special importance
They are not used for generic names.
Let's go through these. If you don't use capitals to begin your sentences, or for people's names, or for "I", in the manuscripts you're submitting or the books you're self-publishing, then you have more issues than I can probably help you with. I can understand why, if you're in a casual environment like social media and typing fast or on a mobile device, you might skip these capitals, though ideally it's best to appear professional at all times (people do judge your writing by your social media communication, if you're an author). Let's move on from those straightforward cases, though, to something with more complex rules: people's titles.
Titles, Terms of Address, and Capitals
If you're talking about "the king," "your mom," or "the colonel," you don't need capitals. However, if you're talking about "the King of England," "King Henry" or "Colonel Baird," you do - both for the name and also for the title. Basically, if there's an element (whether it's a placename, like "England," or a person's name) that has a capital, and a title's associated with it, the title gets a capital as well. (In American usage especially, you can get away with dropping the capital if the title occurs after the name: Michael Bloomberg, mayor of New York City.)
The title also gets a capital if you're using it as a term of address, since it's standing in for a name. "Hi, Mom!" requires the capital. So does "Good morning, Colonel." They get capitals when you're talking to them, but not when you're talking about them (using their generic title and not their name): "Let's ask my dad. Hey, Dad!"
I'll digress briefly here to talk about how you address someone with a title. I read a lot of fantasy fiction, which tends to have medieval people in it, and they often have titles. I frequently see the titles handled incorrectly.
A king is "Your Majesty". A prince is "Your Highness". (Not the other way around, and I've seen both of them swapped over.) A duke is "Your Grace". They don't get addressed by the name of their title. Prince Charles has been known to joke that when he's in America and people address him as "Prince" he feels like one of the Air Force mascot dogs.
A knight or baronet is addressed, and referred to, as "Sir Firstname," or referred to as "Sir Firstname Surname". He is never, ever correctly referred to or addressed as "Sir Surname," a mistake I've seen in more than one published story.
If you're going to include title-holders in your fiction, take the time to check into how they should be referred to and addressed. The Wikipedia article on Royal and Noble Styles is a good starting point, but consider people like bishops, ambassadors and so forth as well. Part of giving a sense of time and place in your story is getting things like this right.
The usage differed from country to country and changed at different periods, of course, but unless you're writing fiction set in a real place and time in our world that isn't post-18th-century Britain, if you stick with current British usage you'll probably be all right.
Regardless of your feelings about religion, if you're using God to refer to a specific deity (even one you don't believe in), it's a name or title, and should be capitalised. If you're talking about a god, using the term generically, it isn't. The use of a capital when referring to God or Christ by the pronoun He is falling out of use, even among Christians, but I would use it if I was portraying the speech of a devout believer or someone from earlier times.
The Bible, the Quran, the Torah and the Diamond Sutra all get capitals, but if you're using "bible" generically (the show's story bible) it doesn't.
Places also get capitals, and the rule is similar to the rule with people's names. Refer to the river but the Mississippi River, the ocean but the Pacific Ocean, the lake but Lake Placid, the desert but the Sahara Desert. I often see writers miss out the capital on the ocean/river/lake/desert/plain/mountain/street part, and that's not correct if you're referring to a specific place by name. Likewise Lincoln's Inn, St Margaret's Church, Cambridge University, but the inn, the church (if you mean a building; the Church is the organisation as an abstract whole), the university.
The North is a place, but north is a direction, and doesn't get a capital. The West Bank is a specific place, and gets the capital, but the west bank of the river doesn't, because it's a generic place--many different rivers have west banks.
Now, you can make a case for capitalising, for example, "the River" if this particular river is extremely important and everyone knows which one you mean when you say "the River". That's fine. But don't deny it its capital when it's part of the name of a specific river.
Names of Things
A similar rule applies with very important items, like the Ring in Tolkien's Lord of the Rings. (Watch out for Fantasy Noun Disease, in which you capitalise every second thing and fill your sentences with strange names and titles. You'll lose most readers very quickly). Official names of things, like the White House, the Department of the Interior or the Royal Canadian Mounted Police, get capitals (though not for minor words, like "of" and "the"), but generic nouns like department and police don't.
Generic nouns in general don't get capitals: a glass of vodka, a brougham, an oak tree. (Don't laugh, I've seen all of those capitalised.) If you go around capitalising nouns all the time, you sound either German or 18th-century, which is fine if that's your intention, not fine if it isn't.
Generic nouns don't get capitals, but trademarks do, so a glass of Smirnoff, a Xerox copy, an iPhone. Use the spelling and capitalisation that's official for the brand, which means if you're not sure, look it up. (If you are sure, it's still a good idea to look it up, in case you're wrong. When I worked as an editor, my mantra was, "Always check everything, even the things you think are right." It saved me from several embarrassing errors.)
Languages and nationalities get capitals, even when they're not acting as nouns: Asian cuisine, my French teacher (but my geography teacher), the Greek alphabet. Religions, also: a Jewish skullcap, Buddhism, he's Presbyterian.
Specific days get capitals: Easter, Christmas, Remembrance Day. Seasons don't: summer, winter.
Some common abbreviations, like i.e. and e.g., are conventionally written in lower case (without capitals). Others, like OK, are conventionally written in capitals. (You can spell it okay, without a capital, but don't write it as ok. That looks particularly odd when you start a sentence with it and the O is capitalised.)
You'll notice that sometimes abbreviations have periods in and sometimes not. The trend is away from using periods in acronyms (IBM, not I.B.M.--especially since it's no longer an acronym for International Business Machines, just a three-letter name). If you use abbreviations, check on the usual styling in a style guide such as the Chicago Manual of Style (CMOS), or in a dictionary. Follow the most common practice, which will be the first one listed, and be consistent, and you'll be fine. This is general advice for everything that isn't covered here specifically, by the way.
In general, the rule for capitals is:
If it's a generic term, and not a name of something specific or standing in for a name, it doesn't get a capital. If it's part of a specific name, it does.