If it is unidentified, unless it is a weird bird, it is most likely a vintage mechanical machine, either (A) Generic copy/clone of a Singer model. If so, then view Singer models to see which looks most like yours, then search for that manual, most vintage Singers have free manuals on line (from Singer, ISMCS, or other collector's web site, or for sale), or (B) Non-clone precision Japanese post-war sewing machine. These are basic machines, and many vintage singer manuals will suffice to be able to operate them. Start by viewing manuals the most probable suspects:
1. Singer 15 (15-Class is the most common clone, and has the bobbin underneath, using a Bobbin Case. The bobbin has a series of holes around both sides.
2. Singer 66 (66-Class is also copied, and has a drop-in bobbin,
not need to load a bobbin case. If the machine is small, you may want to view manuals for the Singer 99-13, a 3/4 scale of the model 66.
3. If these two do not help, then just start looking for images of machines that look like yours, and review their manuals. You will soon be able to figure out how to thread and sew on most sewing machines.
Good place to start to ID machines:
-- Amender (2016 FEB 20)