The basic url looks like
https://twitter.com/intent/tweet?text=<your_text>&url=<url_of_page>
The rest is upto javascript. For
You can save this script as a bookmark, and access it anytime. In fact, you can drag the below link to your favorites bar (on IE) or bookmarks bar on (FF or Google Chrome).
Share page
There are a host of other parameters for other various ways in which you can tweet, but I am stopping short here. You can explore the documentation for learn more.
Ps: These things won't work inside an IFRAME