show all categories

hello,
i’m totally new to wordpress. so i need a little help to accomplish my blog. i have created my blok with 10 or more categories and in every category there is about 4 or 5 subcategories. i want to show all categories underneath every category its list of subcategories. whatever it has posts or not i need to preview every category and its subcategories underneath. if there is a plugin or a snippet of code that make that function, so i will so thankful.