Check the model number of the machine. White sewing machine manuals are available from Singer at http://www.singerco.com/accessories/instruction-manuals
. Enter the model number and click Search.
The White Sew E-Z model W338 manual is available here: http://www.singerco.com/uploads/download/705_white-338-x.pdf
Unlike some machines this one doesn't really need the bobbin threaded. Place the bobbin in the bobbin holder and leave about 5" of thread extended out from the bobbin holder. With the bobbin compartment still open, lower the presser foot and turn the handwheel to move the threaded needle through the stitch cycle. (I find holding the threads helps; normally, you'll feel them pulling under your fingers.) When the needle has returned to the top of the path, lift the presser foot. Then pull both threads around to the front (the bobbin thread should wrap around the top thread). Then move the threads to the back of the presser foot.
If the needle isn't catching the bobbin thread, first try a new needle (bent or dull needles don't always function properly). Next check the bobbin holder for nicks or other damage. There is a small hook on the bobbin holder that should be where the top and bottom threads intertwine. Under rare circumstances, you will find that the bobbin tension of the bobbin holder is misadjusted. This can interfere with the bobbin thread twisting properly. There is a little set screw that allows you can tighten or loosen the bobbin tension as needed. Adjusting the screw is to be avoided when possible and done in small increments (1/8 turn) if needed. It's easy to strip the threads for the set screw and end up needing to replace the bobbin holder.
I hope this helps.