Hi there, welcome to FF&W. First of all you have chosen an interesting and not-so-popular theme in the form of PMD. It can work either ways, the originality of the story might attract people or if it isn't decent people might not even consider reading it. So it is up to you, and how well you write. On that account I wish you a good luck.
Going into the actual story...what you have posted in real sense is not a story. You know, you are just telling us what happened, as if you are writing a summary of events of what happened in a diary or something.
I woke up on the beach called Naniki Beach. I looked around, And I found a shelter, with a Pichu in it.
I presume the story is being written in Eevee's point of view, then how did Eevee know that it was in Naniki Beach? Also it would be better if you actually describe what is happening, like the surroundings.
While doing so you should put yourself in Eevee's shoes, now you wake up suddenly in a place you have never been earlier, what will your reaction be? Do you just get up and find a shelter?
Just then, a Misdrevious came in and stole the bag Pichu had. and ran into the forest near the beach. we ran in and decided to get the bag back. we fought, and we beat that misdrevious. she ran away, and we got the bag back. we went to she shack, and we rested up
Once again this is like a list of activities I have done today rather than a story. You could have at least described the battle or the forest or basically anything else.
I hope this does not discourage you from writing..if you are really interested in posting a story with minimum descriptions there are people who can help you with it. So please don't hesitate to ask for help.