SSL - Building SSL website

If i get a company to build a website for me and I tell them I want it SSL compatible for everything like user logins etc, do they need to know my SSL certificate details OR once they build the website do I just install my SSL certificate on my server and my website will automatically use SSL without me adding any code?

They don't need to know any details of your SSL certificate. They will build a website for you and you can just deploy it on SSL enabled web server without any changes to the code.

No, they don't need to know your certificate details.
Most webservers will put a enviroment variable, when the server is running secure.

So if the company that develops the homepage wants a page only accessible in SSL, they will make the code check for this enviroment variable, and if not, the page will error out.

But if you want your user logins and such accessible both from HTTP and HTTPS, your website company does not need to care about this, all this are handled by the webserver.

The website company only needs to build the page like any other HTTP page.

Just to expand a bit, a user should be able to get to your login page from anypage, SSL or not.

If you have the user enter a username and password, or anything similar, they to have any claim of security, you have to do the POST using SSL.

If you pass the username and password in the clear, you are open to all sorts of attacks. Don't do this.

How is this done?

In your form's ACTION line, use HTTPS://mydomain.com
rather than http://mydomain.com