Add New Content Database to SharePoint 2010 Web Application

Step 1)   Go to "Central Administration"-->Click on "Application Management"--> Under Database section-> Click on "Manage Content Databases"--> click on "Add Content Databases"--> Enter details.database name, server ect.,

Step 2) Create Site collection on New Content Database
before that remaining content databases should be brought to "Offline".

Step 3) Click on "Content database name" --> Change Database Status to "Offline"

Step 4)   Now the First Content Database Status is  "Stopped"
   content database list as shown below

Step 5)  Now click on  Application Management(in Central Administration)--> Under Site Collections-> Create Site Collection-->

Select  appropriate "Web Application"  as u did in Step 1)
Enter some title,desc, Enter site collection URL, enter administrator name ,leave remaining things as default.
Click on OK

Step 6)  Now goto "Application Management"--> Under Databases section--> Click on "Manage Content Databases" -->select appropriate Web Application. in this case http://sp2010:27657

First Content Database "WSS_Content"   has  3 site collections
Second Content Database "WSS_Content_Contentdb2" has 1 site collection.

Note: Once new site collection created, first content database can be made "Online".

All new site collection data will be stored in content database, which is online,at the time of creation.

User can Query using Server Object Model--> Web Application,and their content database using
following link Query WebApplications & their content Databases ....