how do I fix my brother XL 2120?

I have not had this brother machine for a too long (I had it given to me by my aunt recentlyso got it second hand). It is the brother XL 2120 model.I don't have a 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 and when I sew it makes a clanking noise too.
I dont have too much knowledge of sewing machines, 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. I found that it worked ok (sounded right too) without the bobbin case. It looked like it was ok 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.

Nabeela
May 2008