If you're referring to the actual design of the interface, determine what your goals are for the site and go from there. Build a mood board with color pallates and fonts, photos or whatever, and come up with a concept and integrate it all in a sketch or a mockup in Adobe PS. Code it like you want it.
If you aren't even at that stage yet, choose a framework and method of building your site that's most appropriate to your level of web-building sophistication and resources you're willing to devote to the technical aspects of your site. You may want to work within a framework or design totally from scratch.
In either case, planning is how you'd start your web design.