Whereabouts are you Shygirl? I'll cheerfully take a look for you if you're anywhere near me.
A few pointers though:
If you're getting tangles on the bottom of the fabric, that's usually due to the TOP thread tension, not the bottom. Try putting a different colour of thread in top and bottom and see what happens. (My bet here is that you've got the top tension too loose, but it's all a matter of balance really).
Start by doing a simple test on the bobbin thread though, just to check: Put the bobbin in the holder, thread it, then dangle the bobbin from the thread in mid air if you can, as if it was a yo-yo. The tension will be about right if the bobbin hangs in mid air when your hand isn't moving, but starts to let thread out if you jerk your hand upwards a bit. If the bobbin falls without you moving your hand, tighten the tensioner screw just a little, but be aware that this adjustment is usually very sensitive.
For the top thread, make sure it's definitely threaded correctly, and that you've threaded the needle in the right direction. Then it's just a matter of adjusting the tension until the threads meet in the middle of the two pieces of fabric. If the top tension is too high, the top thread will be a straight line on the top of the upper piece of fabric, and the bottom thread will be pulled all the way through until little loops of it are also visible on top.
If the top tension is too loose, you'll be pulling too much thread off with every stitch, and this ends up underneath the fabric in big loops, and can also get caught up underneath.
So, summary:
1) Put different colours of thread top and bottom so you can see what's going on
2) Check bobbin tension is correct by doing yo-yo test
3) Check machine threaded correctly on top
5) Check top thread tension and adjust until the loops meet in the middle of the seam, i.e. between the two bits of fabric.
I hope that helps - let me know how you get on
.
Womble.