How do I find out my opponent's HP and stuff?

<font color=ACEACE>HP reall doesn't mater in the damage Calculator, though you have to check out the Psydex on the main site, it has all Pokemon stats (including HP)

Here is the site:

http://dynamic3.gamespy.com/~psypoke/dex/index.php

Though finding your opponents HP is difficult because it depends on their DV's and EV's. You just have to look at it's level and guess. Around what it would likely to be.