No one knows your company like you do and so your site will really benefit from your input in this way. At the same time other materials such as promotional fliers and posters etc can help your website development company to get a better feel for you business and how the website should look as a result.
As soon as you know you need a new website design then, start collecting relevant material that you can give to your designers. This might be things such as leaflets and fliers, as well as photographs of relevant things relating to your company and staff, cuttings from newspapers, reviews, statistics, bios of your key staff etc etc. At the same time start knocking together some content – some FAQs for example, an introductory paragraph or two, a bit about your company, and any other writing you want to eventually find its way onto your web page.
Other than that the rest will be left to your web design service. Simply kick your legs back and wait for your new site to be developed.