Nobody Needs a Website
What you need is a way to attract and serve customers. The right website can be an effective tool for doing that. The most important question to ask when deciding what type of website to develop is "What do I expect this to do for my business?". It may be as simple as allowing your customers to check your hours and location or it may be creating an entire online enterprise.
Content is King
No matter how big or small your online presence is, the single most important thing is quality content. You can spend as much as want on getting your site high on the search engines (more about that later) but if people don't find value on your site they won't stick around. The other thing about quality content, it has become the single biggest factor in search engine placement. That is why it is important to work with a professional who can develop the content and content strategy.
optimize, socialize and anaylyze
There's search engine optimization and search engine marketing (SEO/SEM). It's sort of like playing the piano, the basics are pretty simple but it's really hard to do at a professional level. How much SEO you need will depend on the nature of your business. If you are mainly reaching out to people who already know you, you don't need much. If you're trying to build an international retail business, you'll need a lot.
One of the biggest changes in the last few years has been the development of social marketing. Things like Facebook, Twitter, Pinterest and other social networks. It's also blogging, participating in online forums and blast emailing. There's always cool new things to keep up with that will either help build your business or suck you into a time-wasting vortex (maybe both).
How ya doin'? No, seriously how are you doing? To find out if your efforts are working and figure out how to improve you need good analytics. They will tell you who is coming to your site, what they are doing while they're there and why they leave.
And back to the most important question, "What do I expect this to do for my business?".