I finished my first Project, which was luckily very easy to start with. Now where I have finished I realised, that it is not very responsive on smaller devices. I am scared now, that I have to change everything all over again. What would be the most basic approach to fix this?
Also I am not really sure how to publish the web page.
@MartinZ, some things to help with responsiveness.
Do not use fixed dimensions on elements, use max-width/height and relative units like %, em, rem and vw to keep everything responsive.
Start by styling for a narrow view port first. Narrow your browser as far as it will go and style the page so it looks good at that narrow width. Then gradually widen your browser and use CSS media query break points to adjust the style for wider view ports if needed.