Well getting good at fanfics takes alot of practice. First you should just experiment with ton's of different ideas and remember once you come up with a story its alright to change it as many times as you want.
Another thing to do is be very discriptive without describing too much (I know it sound's complicated). Let each character have uniqe traits and let the reader feel for them by talking about them and what they go through.
If you're going to have a Character die then let the reader get to know feel for it more than the others and right before its death bring out the Characters personality more. I have more tips if you wan't them just PM me.
