What would have to be done to make a story enjoyable an not too cliche?
Most of what makes a story enjoyable are its characters. So long as the characters aren't too cliche, you've got the basis for a story. Now, if your story is based on an event, then what you're writing is a description, rather than a story. I can't think of any stories I have ever read that did not have characters, even if the characters weren't sentient.
How would I make the characters more believable?
I've found one of the key part of writing believable characters is to give them aspects that you can relate to. he problem with this is there are going to be characters you simply can't write for. This also requires a bit of introspection, and thinking outside your comfort zone. If you want to write for a homicidal maniac, you have to think like one. Go to the dark places in your mind, and come up with the motivations for his hideous crimes. By the same token, writing for a character that loves live requires you to fill your heart with joy and seek the best in life.
How would I actually start out a story?
"It was a dark and story night..."In media res
is actually a good way to start a story. Drop your reader into the middle of something exciting, and build the story from there. But come back to the events leading up to that moment. Don't start your characters in the middle of a fight scene, and then never explain what it was about.
Alternatively, start with flowing descriptions of the scene and zoom in on your focus. It helps to think of the opening of a story like the opening of a movie. Action movies start out with a chase. Period pieces start with spanning shots of the country side. How you start your story depends on the plot.
How many pages is a chapter supposed to have?
A matter of preference, as suggested before. If you're writing an actual book for publishing I'd recommend 4-6k words per chapter. For internet publishing, 3-4k is probably sufficient.
I'm worried about messing up the tense and make a confusing story, how would I avoid that?
Tense is tricky, no doubt, but a good rule of thumb is keep the same tense throughout the sentence.
Doing that will make your life easier. In general, keep the same tense throughout the story. Is everything happening now? Keep present tense. Are you relating a story that has already happened? Keep everything in past tense. Remember your past/present perfect tenses can help change the rhythm of your writing without falling out of tenses.
Some other tips I've found that make stories better:
Read your story aloud back to yourself. Don't read it as if you wrote it. Separate yourself from the story, and read it like an editor. I found that printing off the story and going over each word with a red pen helps me track down grammatical errors.
Don't repeat the same words in a paragraph. The, a, an, and pronouns excepted, repeatedly using the same descriptors for things is boring, and doesn't give the read an idea of the details. I try to add variety to the character references as well. Say you've got John. John is many things, and we can refer to him as such: the butcher, the madman, John, him, he. Also, don't start your paragraphs with the same word repeatedly. Starting every paragraph with "I" or "The" smacks of lazy writing.
Speaking of Paragraphs: Break them up. Walls of texts are not conducive to good reading. Limit your paragraphs to half a page. By the same token, paragraphs are longer than two sentences, unless someone is speaking. A different character talking gets a new paragraph. That's how it works in English, just freakin' do it.
Dialogue: break it up. He said, she said, blah blah blah. Use different descriptors. Characters don't talk in monotone to each other. Find a book of synonyms (Protip: Thesaurus? Your new best friend.) Characters can ask, reply, yell, holler, pant, gasp, whimper, cry, fume, lilt, sing, smile, grin, cough, gag, and a whole range of other things. Make them do those things to set the tone of their speech. "How dare you!" she said vs. "How dare you!" she gasped.
Another, typological thing, is, to please learn how, to use commas. Commas are the garbage bags of a sentence. Anything in commas can be left out. That doesn't mean it should be.
Some of the best stuff is in commas. They also act as conjunctions, which means to join parts of a sentence. You can add incomplete sentences to another by adding commas. This makes them clauses. Comma are tricky. Use them to often and your sentence suddenly doesn't make sense. Use them not enough, and you're just rambling. My three basic rules for commas are as follows: They indicate a pause, they describe a thought or action, and the conjoin clauses.
Writing is technically challenging. I assume that you natively speak English, which a complete pain in the ass to get grammatically perfect. There are ten different uses for a comma alone, not even considering things like the apostrophe, or semicolons. And that's just the technical aspects.
Writing for emotion and enjoyment is even harder. A technically perfect story is still boring if nothing interesting happens. Stories tell about the happenings and their effects on characters. A well written story can make the character come to life in the reader's mind, and challenge the way people think about their lives.
It's my belief that everyone can and should write. We all have stories to tell. Write, and let no one dissuade you from telling yours.