Realization of the website is normally done by an external bureau. Rolling out the website in parts will give you the opportunity to test sooner and prepare content for going live earlier. It will also give the builder the opportunity to solve blocking issues earlier.
After being confident that the website is good enough to go live, you should start setting up a script with things to do until going live. Depending on the size of the site and your organization this can become quite a task. Also search engine optimization and webmarketing aspects come into focus now.
If the website is part of the coreprocesses for your company consider a soft launch, making the website available for a certain group of people before going live 100%, giving the site to work some time with real data and real situations, but with a lower volume and without the risk of stopping the company.