How to start a Web application with Asp.Net

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Windows

hi,

How to start developing an ASP.Net application in very Professional way?


for example,

For my database queries like select,Insert,Update...etc, i can simply use it as

sqlconnection con=new sqlconnection("...ConnectionString...");
con.open();
sqlcommand cmd=new sqlcommand("---Some Queries--",con);
con.close();

But i dont need it.

I want to know, how was an Corporate applications runs these above code With full security ??

While i surf, i came to know about "data access layer"...
I need to know, how to start developing an SECURE web application????