With most seemingly daunting tasks, the best approach is normally to break them into smaller tasks which seem less daunting.
It would make more sense to break it up by functionality. You say that you "literally have no idea on how to start it". If that's the case, start where a user would. What's the first page your user would see? Would it be a login page, a home page with lots of links to different content or functionality, or something else? Then do that first, and ignore everything else you need to do. Leave dead links on the page to non-existent content, leave the password reset link (if there is one) broken. Just get something, anything, displayed. The simpler the better.
Then write down a list of the things which need to happen next. Maybe you want to add a page which sits behind one of those broken links, or maybe you want to make it look better by adding some CSS. Whatever you choose to do, make it simple and manageable. Once that's done, move onto the next thing, and then the next thing.
Every long journey is just a sequence of steps, but often the first step is the hardest. Unless you start somewhere you'll never get anywhere.