I will try again to ask my question. I hope i can explain it better.
Firstly im following the tutorial https://cgcookie.com/blender/lessons/part-01-sculpting/ in that i understood that u can select the edges next to what ur mirroring and with a simple command ( which i thought was crtl c) its possible to center the edge to the middle of what you want to mirror. Because this dosent work with me i wondered if i got the command wrong. He dosent explain it in the video but just shows it with screencast keys.
secondly The problem with my object it that on the front side the mirrored edge fits rightly at the back of the figure it has a rim, so dosent fit together. How can i get the back and front level so the both fit well. here my blend file.