We've talked about themes and features as groupings of work. These are really useful constructs to help us get organized but they're still too big for a team to work on and deliver value in small timeframes. The user story then is the tactical level of work that can be delivered quickly. At the same time, they're not so small that they deliver no value.
Anyone on the team can write user stories but it's usually the PO. As the stakeholder and representative for the customer it's their duty to ensure everything in the backlog adds value for the customer so they usually write the user stories.
First, good user stories are independent. It can be delivered separately from other stories and have value by itself. If you only have time to do one thing, this story can stand-alone.
User Stories are also negotiable. Until the story is committed for work, it can be rewritten, changed, or cancelled at any time. User stories must be valuable. It delivers value to the PO, stakeholders, and customers.
User stories should be meaningful and it should deliver value to work.
You must be able to estimate the size of the story in story points. That means that the story is descriptive enough so you know what has to be done to finish it. Only then can you understand the effort required.
A Story is a task, with all the details about the task mentioned in the story description. The story is then assigned a time(story points), within which it must be finished. It is also given a priority, important tasks can be marked high priority to be worked upon first.
There are many softwares available these days, for complete Scrum Planning like Jira by Atlassian.
Writing good user stories can be challenging but if the requirements and planning is good, it gets easier with time.
Throughout the agile lifecycle, your stories evolves. Large stories can be split to smaller stories, if the single task is large to be implemented in a single iteration.
Multpile stories are clubbed under one Epic. When we say, at times, a big story can be disintegrated to multiple small stories, we group all the small stories under one Epic. It is like one big story.