Modern mirror glass is actually coated with a material made from a mix of different metals, you can see this sometimes as black spots where little bits have fallen off the surface, so creating a highly reflective metal such as chrome would be the easiest way to go.
To create a chrome shader use a Blinn at default settings and set reflectivity to 1. In order for this shader to work and allow for reflections you'll need to have lights and raytrace the scene, if not this shader will render black. Raytracing is enabled in the Maya software tab under Raytracing quality.
How the mirror glass will look depends entirely on the environment it's in, so this will be determined by the lights and other materials that are present in the scene. A mirror itself in isolation would have nothing to reflect, whereas if you have lots of different colored objects in the scene something like this will come out a lot nicer.
Different metals will have different values of reflectivity, also they have a very low diffuse values and recieve most of their illumination from their specular highlights so keep this in mind.
As for the specular settings they're dependent on your scene, a specular highlight is a reflection of a light source so you want to tweak these settings to match up with the distance to the light source if it's visible. In the case of a mirror you often get very large and wide areas of specularity if the scene it's in is bright.
If you render this in mental ray instead you have the option to blur the reflections as well, if you go down to the mental ray tab in the attribute editor for the material you'll find a setting called mi reflection blur where you can adjust this. In the real world no reflections come out without at least a slight blur, as for a mirror you'd probably want reflections to come out quite sharp unless it's an older mirror or you're going for a special effect of some kind. Mental ray can give you some better results as you have architectural materials specifically designed to create things like glass and metals, also they include a lot of presets, but these materials are a bit more difficult to use than Maya materials such as the Blinn as they're a lot more complicated. But if you want something photoreal you would need to go with mental ray. Another advantage mental ray has is that you can use image based lighting, which allows you to create some really nice looking reflections on an object.
Again, how a material reflects light and what kind of specular highlights it will produce comes down to the way it was created in nature so to get some really nice results in terms of photorealism you'd need to take the different properties of the metals that go into this metal coating into consideration, as well as the shape of the geometry you're applying the mirror glass to. Reflections and specular highlights are also view dependent in the sense that they will change depending on the angle between the material surface, light source and the camera, so they will move as you move the camera in your scene.
I hope this helps, it's quite difficult to find information on the topic of shading and how to create basic shaders online, I know I've struggled a lot with it in the past. If you want some more in depth information there's a really good book called Advanced Maya Lighting and Texturing by Lee Lanier, it takes you from the very basics to some fairly complicated stuff and has quite a bit on reflectivity. As well I've just finished making a tutorial on practical lighting and shading which goes over how to create shaders for things like glass, shiny and brushed metals, satin, plastic, porcelain, matte fabrics etc. because it's actually quite easy to get some really nice results and as I said before finding good information on this subject is difficult unless you turn to books. So you might want to check that out if this is a topic you'd want to learn some more about, it should be up on the site tomorrow.
There's also some good lighting and shading blogs, most of the things I read are on 3ds max but if you know the basics it can be applied to Maya easilly enough. I'll have a look later and see if I can find the links and post them up for you.
Best of luck with the mirror!