I have already installed the EPiServer CMS7 and had the sample Alloy website running in my PC already.

Now I am just wondering how I can create a e-store sample site which can manage products(is EPiserver Commerce the best choice in this case?). 

Thanks!

If you have isntalled EPiServer Commerce on your PC, then you should able to install Enoteca sample site via Deployment Center.

All Actions -> Installed Products -> EPiServer CMS -> Version 7.0.586.1 -> Modules -> Install Enoteca Sample Site 7.0.243.0

You will be asked for existing CMS site which will be used as target site for Commerce module.

How come it said "Full text searching component is not installed " ... ?

How to install it ? 

Try these.
http://msdn.microsoft.com/en-us/library/ms142490(v=sql.90).aspx
http://www.techrepublic.com/blog/networking/adding-sql-full-text-search-to-an-existing-sql-server/5546

Thanks K. 

http://www.techrepublic.com/blog/networking/adding-sql-full-text-search-to-an-existing-sql-server/5546

This link was what I found as well. But a bit confused what is the "installation media "?

Because when click "Add", a "Browse For Folder" is shown up and the message is "Browse for SQL Server 2012 installation media". 

Is that the folder containing the SQL Server installation .exe  or .iso file I downloaded online? Or those files stored somewhere in C:/Program Files ... ? 

Installation media usualy refers to image/directory from where software was installed. Try to mount .iso file to virtual disk and browse to that disk when asking for media.