Hi , yes it is very common to model half a model and then mirror over, then when complete if it is say a human character you might want to add a little non-symetry after you have the basics modeled and duplicated.
ok the first thing you have to be sure of in order to mirror is that your pivot point is in the centre of where you want the mirror to occur i.e if your model is at the centre of the grid so that half is on the + axis and half on the - then your pivot needs to be dead centre between + & - on the centre line.
To move the pivot point if it is not already in the location needed you hit the insert key on your keyboard which will allow you to move the pivot without moving the geometry.You can snap your pivot to the centre line by holding down the x key and middle mouse clicking on the center of the grid,once your pivot is where you need it to be hit the insert key to complete the repositoning.

To mirror your geometry first select it and then go to the edit tab and click on the option box for dublicate. Adjust the number to -1 in the x scale box ( note if you want to scale in a different direction choose the axis you need insted of x ) then hit duplicate.

Dont forget that if you have mirrored half of an object then you will need to combine the geometry and merge the verts to join the two halves together.

hope this helps ..goodluck J.S