How can I fix my Brother sewing machine?

I've had this Brother sewing machine for about 15 years using it only occasionally. I can't find a model number on it and have long since lost the instruction manual for it.
It keeps jamming up when I try to start sewing. The thread on the bottom of the fabric is all knotted and the sewing machine locks up after only a few seconds.
Forgive my lack of knowledge for sewing machine terms, but what I did was, I removed the plate underneath the foot and needle and removed the bobbin casing. I took a can of air and blew it out. I also took a little brush and cleaned it out that way. Then I oiled it sparingly since I wasn't sure where the oil should be placed. Once I put it all back together again, the same thing happened.
I took out the bobbin and watched what was happening when the needle dipped down. The thread from the needle was pulled around the outside of what I think is the bobbin casing and this looks like what is causing the jam up. But I can't be sure because I don't know what actually is suppost to happen.
Is there any way I can fix this myself?
Thank you so much for your help.

Nina
March 2008