Change the menu of the theme from horizontal to vertical without Using plugin

Hi, I have this template theme which I have to change the menu from horizontal on the top to display vertically on the side bar where at the moment it displays blog categories, which i will get rid of since I wont be using. How can I do this with out using a plugin? It just needs to display vertically on the sidebar so I can edit its css to make it look the way I want it to look. Any Suggestions??