lol don't worry, i'm already not finishing a bunch of stories
just write! like mentioned below, stick with oneshots first. that's what i did, and i finished them (trust me, if i can finish them, you can too)

Write the story you want to write. Note the ideas and try to order them. Then begin to write. Focus only on one story with the goal to finish it. So make your story simple. A one-shot as Casely say or not. My first fanfic was a good story of 20 chapters! And on contrary, now I am best in one-shot. I think my passion for my first fanfic was so high that I manage to do it. SO I will end with: begin to write and try!

Stick to writing only oneshots for now. It's easier to come up with a single chapter than it is to come up with many.