I'm using Drupal 7 and want to add meta tags in my theme. How can i do it without any module?
Using a module is the preferred way, as in that way you don't need to edit a theme, or create a sub-theme to set meta tags.
The alternative would be changing the html.tpl.php template used from the theme, and add a call to a function present in its template.php file that returns the meta tags to add to the page.
There is one case where editing the theme being used, or sub-theming it, to add the meta tags would make sense: when the meta tags are very specific for the theme being used that they should not added to every theme used from the site.
I would also prefer using a module as I would not need to worry about updating the module for a new Drupal version, or writing the update functions.
If, viceversa, I need to be 100% sure there will be a module version for the next Drupal, I would write my own custom module, knowing that it's just me that can fix any security issues I would introduce in the code (which also mean I am left alone with finding any security issue).