We are not experts, although we have some knowledge from writing and also studying. However, what to us is most important, is to keep ones writing style. When the readers read a book of yours, they should recognise themselves. It should be your text, not someone elses.
With that said, do your best to keep a high tail guide. Nothing can destroy a good story as much as sloppy writing. If you are not sure when to use indents, how to use punctuation and so on, learn! Then there's spelling. When we write, especially when writing fast, it's almost impossible to never make mistakes; "and" becomes "adn" and so on. Check carefully and always use and leave spelling and grammar control on. However, don't trust it completely, it's a computer checking, not a human eye. That eye will be yours or someone else that you trust with your text.
There are so many things one can say about the art of writing, but the best thing to do is to write, write some more, and then write more again. Insecure? Ask someone to read and give feed-back. You can for example find facebook groups with test readers. Trust them with as much as you feel comfortable.
Another thing to be aware of is how easy it is to use the same expressions or the same words. Do a search on some common ones and you will probably be surprised how often you use your favourite expressions.
Keen to start writing but you don't dare by the way? Remember that we have all written our first novel, short story, poem or whatever for the very first time. Maybe this is exactly the time for you to write your story the very first time. Good luck!