No company, organization, or individual should take its website lightly. Your website is the face you show to the world, and these days it's often the first thing your customers, clients, or personal contacts will see about you. In other articles, I explain how to work with the building blocks of a website; here, I'll talk about problems you should avoid in order to build a successful site.
Don't Skip Designing for Mobile
If you're building a new site from scratch, design it to accommodate mobile users. Unless you know exactly how every visitor will view your pages (for example, you're creating a web app for a corporate intranet), assume that visitors might use a computer, a smartphone, or a tablet to interact with your site, and make sure that the site provides a good experience for any of those devices.
If you commit to mobile, you must accept the responsibility of testing your sites on a variety of mobile devices. Buy or borrow an iPhone and iPad (by far the largest number of devices browsing the Web), and don't forget Android smartphones and tablets. You can't possibly test with every kind of device, so test with whatever you can before the site goes live, and then be prepared to tweak the site after it launches.