how-to-make-a-trending-topic-on-twitter-

To trend depends on how many people are tweeting about it.
The frequency becomes secondary , volume is primary focus
But you cannot trend if you tweet more frequently. Tags depend upon certain level of frequency but thats not all, They have to be diverse and collective.
Most people tweet if they get something gratification in return. There can be contest or prize.

Targeting country for your website: Geotargeting

Set a geographic target:

Sign in webmaster

1. On the Webmaster Tools Home page, click the site you want.
2. Under Site configuration, click Settings.
3. In the Geographic target section, select the option you want. If you want to ensure that your site is not associated with any country or region, select Unlisted.