First, make sure you write good villains, don't make your villains just plain old evil, give them a purpose, some endeavour. Villains work best when they are more like heroes than the heroes are.
Give your plot some proper thought, plan it out first, don't do what I did for the first few tales I wrote and just make it up as you go along.
Draft and redraft again and again and again and again and again until you are happy with a piece of work, don't just get to the end and assume it's done.
Don't treat your readers like idiots, no-one likes having every intricacy of a story explained to them through arduous amounts of dialogue or big walls of text, let your audience infer events outside of the main story from what is said within it.
Don't get too bogged down in descriptions. People will form their own notions of what a character looks like, you don't have to do it for them.
Don't take yourself too seriously, especially when writing fantasy. A fantasy which is too serious just becomes conceited. Make the odd joke, have some comic relief. Real life isn't all serious, why should your story be?
Never, ever, EVER, make your protagonist perfect. I cannot stand it when main characters effortlessly overcome every object in their path. everyone has their weaknesses, remember that.
Equally, don't make your protagonist too weak. No-one has sympathy for a hopelessly useless character, they just despise them. Witness whatever her name is from Twilight. *shudder*
The problem with High fantasy is that you always end up copying Tolkien in some way. You will note that a great many of the things I have warned you against here, J.R.R. Tolkien is guilty of. Aragorn is perfect, Frodo is helpless and weak, he takes himself too seriously, his villain has no real goal and his stuff gets boring. (Summary of The Return of the King: Sam and Frodo walk, Sam and Frodo walk, Sam and Frodo walk, Sam and Frodo walk some more, there's a big battle, Sam and Frodo walk, Sauron dies, everything's better.) Do try and be original.
Finally, if a story isn't fun to write, then it won't be fun to read. Don't write stuff you don't like writing about because otherwise you become bored and your story will follow suit.