Hello Martin.
I will assume that you have bought a mechanical camera which can decode DX film speed indications which have been encoded onto 35mm film cassettes. Here is a link to the Zenit site where I think such a camera is identified and described:
http://www.zenit-camera.com/zenit_camera_412dx.htm
As to its operation, it should be the same as for all classical mechanical cameras. You will need to learn the fundamentals of photography. Specifically, you will need to understand the relationships of film speed (ISO, formerly ASA), Aperture (lens opening), Shutter Speed (exposure time), Focal Length (lens' angle of view), how to use a light meter (in the camera in this case), and some basic compositional skills.
The indicator within the window of the camera should indicate when you have a workable combination of shutter speed and aperture to achieve a suitable exposure for your film.
That might head you in the right direction. Ask questions as you go. Lots of folks here are only too willing to help you.
Good luck with it.
