|Basically speaking, you shouldn't fiddle with the bobbin tension, it's very sensitive and really shouldn't need any adjustment normally.
It's not a bad idea to mark the 'normal' setting if it's correct and you plan to change it [a hash mark of nail polish on the screw and the casing...use a toothpick for a little mark].
That being said, if you want to use some chubby thread in your bobbin, or anything thicker for bobbin work [too big to use on top in even your biggest needle], here's how you adjust the tension and know it's correct.
Pull the bobbin and its case out of the machine. Make sure the thread is guided through the case correctly. Now pick up the whole works by the thread. The weight of the case, bobbin, and its thread is just the right amount of weight to gauge the proper tension. When you pick it up by the thread tail, it should just hold. If you give it a little shake up and down, the casing should allow some thread to feed out.
If the tension is too loose, tighten the screw a tiny bit [clockwise is tighter]. If it's too tight, loosen the screw a tiny bit.
Now it's ready to go back in the machine.
Safety tip...the bobbin screw is extremely tiny, if you should happen to loosen it so much it falls out, you may spend the next week looking for it. When loosening, work over something that will catch it if it falls.
Ok, so you forgot...dropped the screw and can't find it. Get your vacuum cleaner hose and slip an old pantyhose leg over the end. Leave it loose so it gets sucked up inside the hose a little. Secure with a rubberband or hold onto the pantyhose. Vacuum the area where you dropped the screw. It will go into the pantyhose so you can easily retrieve it.