Princess Sisi and Schloss Possenhofen

I want to go to the residenz or princess Sissi...do you guys know where is it...i heard it is a lake...forgot the name though.

I think it's Starnbergersee

Yep. Place called Possenhofen
(Site in German)
It's a nice ride on the train and a nice bit of parkland down by the lake but the residence itself is private and you can't go in

Schloss Possenhofen at Starnberger See (lake).
The castle where Sissi grew up is privately owned and not open to the public.
The Residenz is in central Munich.
The Bayerischer Verwaltung der staatlichen Schlösser, Garten und Seen (in English) website will tell you more about castles, lakes and gardens in Bavaria.