ad code in twenty ten single coloum

Don’t edit the Twenty Ten theme. Instead create a child theme for your changes.

thanks, but I am not a software person so I don’t understand much about all these thinks. Just tell me where to insert code

edit onecolumn-page.php in your child theme;
for instance, add the code after this line:

<div id="container" class="one-column">

thanks everybody, it didn’t worked , so I put it in textbox widget it worked perfecty. Can you guide me how to put ad between first nd second post on the main page. Please visit my site https://www.mathsstudy.com and tell me how to improve my site

Don’t edit the Twenty Ten theme. Instead create a child theme for your changes.

once the child theme is active, somebody should be able to help with your problem.