i'm following a tutorial provided here to add navigation menu, but not able to find out set of #nav in my style.css & my theme is eleven40. What we need to replace to get navigation menu?
Follow the other steps specified in the tutorial and you are set.
You should use a plugin like WP-PageNavi, which is also highly customizable. Additional Wordpress plugins can end up in more server requests. But there a ways to prevent it, you should consult How WordPress Plugins Affect Your Site’s Load Time.
So, there is no reason to implement a custom navigation bar.
And please try to avoid such of these question in the future. We won't install your specific theme just for an answer. Try to formulate neutral and general questions. You should check this.