You first need to register a domain name, which can be done quickly and easily with any domain name registrar.
Your URL should be the same name as your business.Try and pick a unique name – you don’t want people leaving out one letter and then ending up at a competing site.
You then need to start putting content on your site. To do this, you’ll need to find a hosting company that will allow you to keep your data, (images, text, etc), on a server. Most start-ups won’t need a large amount of space, so this should be relatively cheap.
Then you need to start designing your site. The complexity of this will vary depending on what you’re putting on your site. If your website isn’t offering any product, but instead just information, store locations, etc, then a basic design will make do, and most web agencies will offer this type of basic package.
However, for more complex businesses, such as those offering online shopping, customisable sites with user profiles and forums, etc, your website design will be far more complex. It would be best to contact a dedicated design firm specialising in these types of sites to help.
These are often a little more expensive, but many provide ongoing support and will be able to fix problems as they arise which will save you time and money in the long run.