SQL Connection Pooling
Essentially this is enabled by default when using the System.Data.SqlClient data provider. There is a very good article about the semantics which can be found here:

SQL Server Connection Pooling (ADO.NET)

Also, you can view the ConnectionString property reference of the SqlConnection class here:

SqlConnection.ConnectionString Property

This contains a lot of information about the options that are available when configuring a connection string for Sql Server. 

You should also note that the actual connection pool itself is kept within the Application Pool assigned to the website/web application, so be aware that if you have multiple sites/web applications using the same Application Pool they will also share the same connection pool which may not be desirable to you.

As such, I would recommend assigning a new application pool for each website and each web application underneath the site itself. You can find information about application pools at the link below:

Application Pools