How to decide where to purchase an SSL certificate and What is the difference between normal and Wildcard SSL CA?

Depends on Your infrasture and requirement. if you want secure channel for many multiple sub-domian and some are  on different servers, then You should go for Walidcard SSL otherwise your choose SAN/etc for2/3 sub-domians. You can buy SSL from OPENSSL, DigiCert, etc. or any local company having Reseller/acount of SSL Provider.

Others detail and configuration can provideed on demand. Thanks

Eventhough Domain Registerars like GoDaddy and HostGators give the facility to purchase SSL Certificates while buying Domain Names. Verisign is the Best Certificate Authority where you can buy SSL Certificates for your websites at Best Price.

And the difference between Normal SSL & Wildcard SSL is that a Wildcard SSL Can protect your Entire URL Combinations. Like, www.techhi5.com, blog.techhi5.com & so on.

You can Buy SSL CERTIFICATES From any Dealer for verisign, Like symantec and also from WebHosting Provider Like Network Solutions - Ipage etc

• Secure Site Wildcard SSL Certificates

Protect the transfer of sensitive data on multiple subdomains under one domain on your server using a single easy-to-manage certificate. 

• Secure Site SSL Certificates

Protect the transfer of sensitive data on websites, intranets, and extranets. Includes up to256-bit encryption

Totally endorse answer added by @Shahin Syed