How to add my html code so as to design the page generated by a view using drupal 7?
In your view on the right-hand side there is an 'advanced' link.

When you click this it will display a number of advanced options.

The option you want is 'Theme : information".

Click on information and you will presented with a number of template suggestions to help you theme your view.

To determine what template to use you basically need to think what level you want to theme at.

Example:
If you click the link 'Display output' and copy/paste the code into one of the suggested file names i.e. views-view--[YOUR_VIEW].tpl.php, this is the top level you can theme at.

If you wanted to theme a particular field you will find that further down in the suggestions.