Bringing extruded plane to one level I have extruded a plane, and I did this all from the front-view.

I am not sure how this could be happen, but when I switch to side-view, I can see that the plane was also moved on some other axis.

It is not an even plane anymore.
Can somebody tell me how to bring it back to one level again and perhaps also tell me how to avoid something like this in the future?
Thank you!
ps: And if somebody could also tell me how to bring the leftmost vertices to one level on the blue axis to allow for mirroring, I would be very thankful as well! Thank you.
I presume, you want to flatten the mesh, to do this select all the faces and press S + Y + 0.  

This should give you the flat plane you are looking for.

In future when modelling, you can replace "Y" with any axis and also when extruding planes, be sure that in the side view the face/edge is flat so that you can extrude worry free in the appropriate orthographic view.