Welcome to the forums, @helennisabelle.
The basics for building a website are HTML and CSS. If you don't already know those, then that's the place to start.
As far as tools go, all you actually need is a text editor, although using a code editor makes life much easier. There are many available, including quite a number of free ones, so it's just a case of searching for one you like. You'll probably also require image editing software for creating images or optimising photos
As @marklenon95 said, some designers like to create a mock-up in Photoshop or similar software first, but others use "wireframe" software, or a pencil and paper, or simply design in the browser. You need to find which method works best for you.