(I am italian, be patient with my english)
+Begin with sort stories.
+Learn also to write as a game, or just to try and see what happens. Never be worry about wasting time or ink: this is very important. If you only want to write GOOD, you will never become a good writer. It's like in a war: you must have some losses. So, write as much 'superbestesellers' as much as 'just to try' short tales made to be wasted, just to see if you are good in writing everything.
For example, try do describe something, and write it. This will make you more powerful in descriptions. Do this for anything you think you are not good at.
Oh, and by the way, never think you are good. Always be 100% sure that 'one day' you will be. This is how good writers think.
+ Try to write an ultrashort story every day. The story only, not all the text. Doing this you will have many bad ideas, but later the good ones will come real fast (brainstorming). Push this to the limits. You wil find how your mind works. Feel it and than master it. Push it. You don't know what your are capable of.
+When you feel you begin to understand overall how writing works, thake the best of the many ideas you had, write something short and complete it. Than wait around one month.
Do NOT re-read during this month.
You don't see mistakes if you re-read immediatly, and if you do, the month wait will be longer. This is really important. Re-writing is as difficult as writing.
Ok, you have waited one month. Do you now see the 'good' and 'not so good' of what you wrote? Ok. Now try to remember: how did you wrote the good parts? Were you thinking to style or story development? Were you writing to make a bestseller o just for fun? Feel the 'mind phase' you were while writing the good parts, and than FORCE you to write again like this. it is THIS that make you better. When you get to do this, you will become a pro.
+ Write what you are good at, not what you want.
+ ok. You have learnt the method: you write good ideas spending many time, and you spend less time to write very short fiction made only to become a better writer.
We have a real problem here: the short 'writing exercise' fiction is better than the works I made to become a bestseller writer.
"I am spending a lot of time on good ideas writing them bad, and a very little time on bad ideas writing them good." it's normal when you are new to writing.
And yes, when you write 'just for fun or exercise', you write better and have better ideas.
When you have accepted it, and you have learnt to ALWAYS write like this, you will become an outstanding writer.