Don't worry about grammar and spelling, that's why they have the red and green squiggly lines on Microsoft word, duh! And really, you are as good of a writer as you think you are. The more effort you put into crafting your sentences and imagining your story, the better your writing will be. The more you write and read, the more ideas will come to you and you will get quicker at finding the words you want to express your ideas. Read novels or works that are the same type of thing you want to write, but beware of stealing ideas. If your English teacher assigns you a project that you are interested in, consider it practice for what you really want to write and put 110% into it. Here's my specific advise on writing stories-Diagram your story ideas out before you start writing. It is easy to get carried away with describing your characters too much, but at the same time you want to know your characters inside and out, so make a chart off all of them and describe every inch of them, just to get your thoughts straight, and only include some of what the readers need to know in the finished work. Before you write the version you really want to work with, write it all out with the first words that come to your mind, and print that out as a reference while you write it carefully. general rule of thumb, the more major a character, the less you should come flat out and describe them, and the more you should reveal gradually through their actions. Minor characters can have a little blurb when they are introduced about what they look like, but only include features that are necessary for the reader to know or that are interesting. An example of what not to do is in The Maltese Falcon, blech. Whenever you are using a metaphor to describe something, always make sure it is something no one has ever use before. If yo ever you catch yourself using a cliche, stop and think of something really creative, and it will make you sound marvelous. ( don't actually use this, but an example could be instead of "she had creamy skin" it could be "she had skin like smooth, perfectly whisked, vanilla rum cake batter")If you are using the third person, use your own writing voice. I'm not saying be conversational, but don't force yourself to use language you aren't comfortable with. Of course you want to sound crafty, but don't use so many academic words it sounds stuffy. Also don't try to sound old fashioned or legend-y when writing fantasy, I could explain why, but i don't want to offend people like Christopher Paolini. Use active voice as much as possible. Have a purpose for writing, chose a lesson you want to teach by this story at the heart of it, so it actually has substance. Beware of Mary-Sues, but don't be anal about it, some characters that have Mary-Sue symptoms are just interesting, but too much is no good.
I keep this book by my bed, it has really good advice and I think all aspiring writers should have it.
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