The main thing to keep in mind with making mobile-friendly layouts is to use percentages instead of pixels (for widths and heights). If you really need to have a set width, then the highest that you can set it to is 300px (at least on my phone it looks fine at 300px). Otherwise, you can also make layouts mobile-friendly by not specifying a width like for background-color for example.

I'm not sure how far your html knowledge goes but if you're still fairly new, then I suggest getting oriented with non-mobile-friendly layouts first to get used to which code stands for what. It really helps to practice with those first. You may need to look at a bunch of tutorials and already-made layouts but I wish you luck and you can do it!