When you design about composition it is good to learn from the classic graphic design info that you can find about print (for example Typography, Grids, Kerning, White-Space, Golden Ratio). They will give you a correct linearity and order that you need in order to design "whatever".
The difficulties in web design is that nothing it is displayed in the same way. This depend from multiple factors: size of monitor, resolution, calibration of colors, operative system, browsers, devices (pc or a mobile), text resize, dynamic pages with ever changing content, forms elements etc...
Your task it is to know about this differences and not fight against it (like many do is called the pre-concept of print design, people expect that what develop has to be the same for every media), but embrace it and adapt everyone of them in a good looking design even if with differences.
Do not forget that in the web concepts like hyperlink, infinite canvas, permalink, interaction flows (like studying about Information Architecture and Usability) Accessibility, web standards, bandwidth, and SEO are really important. You can imagine that behind each word that I used there are tons of books and website specialised to any of this things, do not panic. If you want to start from somewhere, please start from this book:
Taking your Talent to the web of Jeffrey Zeldman (a prominent figure in the web standard nowadays).
And good luck with your Web Design Career