In the builder the page will appear empty and is shown in desktop device overview by default. If you want to build a form specifically for another device type, use the button on the left to switch to tablet or mobile.
To start building your page, press the four squared icon in the top right corner to open the section menu. Here you can find all the different content section types that you need to build your form. In most cases you would start off by dragging your Header and Footer Section into your page, and start from there.
To build out your page with more sections, you can choose the different content blocks under Additional Sections. Here you can simply drag and drop whatever content section you need.