short answer-
put the mouse over the 3d window, hit space -> add -> armature.
long answer-
when you add an armature it will appear wherever your 3d cursor is, which could be anywhere, so the first thing you need to do is make sure the 3d cursor is located at your model's centre (the little pink dot). to do this you first need to select your model with the right mouse button. the you need to open up the "snap" menu by hitting shift-s (very useful tools). then select the option "cursor -> selection". this will put the 3d cursor on the your model's centre. you can now add the armature with space -> add -> armature.
the armature will only have one bone, but that's enough for now.