In simple terms, a shared SSL certificate will validate the ownership of the domain name of the server that is hosting your web site, not your web site. That domain name may be something like serverfive.hostingcompanyname.com and not
www.mydomainname.com.
Depending on the browser's security settings of a visitor to your site, their attempt to access any secured pages on your site may be simply blocked because of your domain name not matching the one in the certificate. A slightly lower setting may notify them of the conflict and give them the option of blocking your site or viewing the secured pages. A much lower setting would display the contents without any warnings. It is up to you to decide how much confidence a visitor has using your site on a shared certificate.
Personally, I would visit a site using a shared certificate that only wanted some very general information about me. On the other hand, a web site better have their own certificate if they expect me to give them credit card numbers and any other information that could be subject to identity theft.
You hosting company may offer certificates to their clients for a nominal fee or you could use a company like
Verisign to obtain your certificate..