I think I know about the surprise party being planned for me...?

Definitely keep your mouth shut girl :) You didn't mention your age, but who knows, it could be some kick **** speed 16 party or something lol. But yea, play it cool/surprised :) That's the best way to do it, otherwise your friends may get mad and/or disappointed that you found out, fights will break out like, "OMG WHO TOLD HER!" and other drama...lol. So just keep it cool and definitely act SURPRISED :)