You can't explain how to develop real use cases in the space of a single forum answer. There's just far too much to cover, and anything that we put in here would be too brief for you to get a real understanding. You might want to read
this[
^] article to give you some idea.
The second part of your question relates to project estimation, and this is based on many many factors, but really needs experience to tackle. How you estimate depends how you are going to run the project, e.g. is it Agile, how many stages you are splitting it up into, what counts as a phase? When it comes to estimation, I find that Project and Excel are my best tools.