I've been studying writing and literature at tertiary level since 2006. I've sat through countless workshops, read thousand of words by other aspiring writers and, quite frankly, written a lot of rubbish myself. Here is some advice I've been given, and also some I'll give to you as well.
From university lecturers:
Write what you know.
I hear this less and less, but let's face it, in some ways this does make a lot of sense. Without proper research, or at least a well imagined world you have created yourself (think Hogwarts), you might either isolate readers or create a story which is completely unbelievable or unfathomable.
If you do create your own world, map it, draw it, write about it first. And then add the characters.
If you don't enjoy reading it, who will?
I'll be honest - I do enjoy rereading what I write. When I don't, I realise that very few people would be interested in it either. Write about what you enjoy reading and speaking about, and chances are others will find it and feel the same.
Don't use cliches.
I have heard this often, but in two of my short stories, cliches have been used to account for the character's inability to communicate beyond phrases they are already familiar with. Mostly though, this advice is fair and should be taken seriously.
Know your audience.
Simple fact is plenty of people are writing things, but often they don't have an intended audience. You can find one or you can change your genre or interest area. It's as simple as that.
Have a twist.
I wrote a great short story one, and my lecturer said "But we decided that every short story has to have a twist! Where is the twist?" Look, there doesn't need to be. A twist needs to work on the element of surprise, and quite frankly, most twists fail at surprising the readers. Do what you want, as long as you end the story somehow. Anything other than "and then I woke up" will suffice.
Truth is stranger than fiction.
Some of the crazy stuff I have seen, heard or experienced is just unbelievable in a story. When you write autobiographically this can come off as completely bogus and you'll lose your reader. Surprises and twists are okay - as young as they are reasonably plausible.
Some things only belong in journals. I've read far too many intimate details in autobiographies of people I know (or authors I don't), and quite frankly, this makes me feel incredibly uncomfortable.
Memories - stop over thinking it.
As with above, writing autobiographically means that sometimes you can get hung up on memories which don't need to be shared. Often these are things which will mean a lot to you, but your readers won't always know your context or relationships with other people in the memory. If you write it down, go back and reread it a few weeks or months later to see whether you have attempted the anticipated impact.
Create characters and keep using them.
I created a character called Jorge, and I keep trying to find places to put him.
I am seriously in love with Jorge. He is named after a guy who was on Australian Idol who thought he was too good for the show and could make it on his own. He didn't. Jorge finished his BA (Musical Theatre) and has worked as many characters including Homer Simpson and a giant termite. Jorge is a jerk, but everyone who reads him likes him because he's crazy and memorable.
Characters you reuse are good because you get to know them better and your audience will have a back story about them too (think Maeve Binchy).
Choose names wisely.
I can't stress this enough. A bunch of names which are either made up, spelt in a variable way or from one or more different countries (other than the country it is set in) can make for difficult reading.
Basically, there are two types of readers, those who memorize whole words and those who memorize sounds within words. I have always only memorized whole words, which makes spelling and reading sometimes challenging. If I am having to learn four or five or twelve new words (even if those words are names), this can be very difficult, especially if they are spelt in a similar way.
The author may have chosen their name for meaning rather than working within the context of the story. Choose carefully young grasshopper.
Write all the time. Lots of what you write will be complete rubbish and you'll throw it out. But there will be some gold somewhere along the way. Cherish it, because such gold is rare and is usually born out of madness, or genius, or both.