Filter the requirements. Whenever you are not sure about something talk to people and get professional advice from analysts, programmers, program managers, or whomever you need. Just make sure that you are aware of the requirements that your test object needs. It might be a good idea to always make notes as you go along.
Next, now that you are aware what the exact requirements are for your test object, you can make a mindmap. A mindmap is an easy way to brainstorm thoughts organically without worrying about order and structure. So please go ahead and download a mindmap software tool of the internet. Or use a free online mindmap web tool. If you don´t like to download anything you can always make a rough mindmap sketch on paper.
Always make sure to keep working on your mindmap so it can develop and grow bigger, with lots of new ideas.
It is now time to start writing the testcase itself. Please note that one requirement can have multiple testcases to verify if everything works well.
It's like a moviescript, you don't fly to Australia to film part 1, then to UK to film part 2, then back again to Australia to film part 3.
It's the same with testcases. It is possible they need a specific chronological order to run/execute them.
For example: It can be the case to test the 'save' button of a website or software tool first. Then if that works you check the other things.
If you test Microsoft Word you'd like the 'Save as...' option to work when you test Headings or Paragraphs.
Once you have created your cases, you can then write a test plan. A test plan in software is the document that outlines the what, when, how, who, and more of a testing object. In general it includes the objective and scope of the tests to be run.
While you are doing this you might want to ask yourself the question: what can be test automated and what should we keep testing manually.