There are several good platforms. WordPress is a popular choice if you want to create your own blog dedicated to short stories. DeviantArt is not just for art but also for sharing written works like short stories. And then there's Smashwords, which is more focused on e - books but can also be a good place to post short stories for a wider distribution.
Well, there are a few places. For starters, you could look at Storybird. It's designed specifically for storytellers and has a nice interface. Another one is Figment. It has a lot of active users and you can get feedback on your short stories. And don't forget about Amazon Kindle Direct Publishing if you want to potentially sell your stories.
First, make sure your story has a clear and engaging beginning. Also, pay attention to grammar and spelling. Use good formatting to make it easy to read.
It varies. Some platforms have auto-posting features enabled by default, while others require manual configuration. Also, it might depend on whether you've given the necessary permissions for automatic posting.