I will echo the suggestion to work with a smaller blade. I don't even stock the #12 blades. A second consideration is the saw speed. Too slow will result in chatter and too fast will make you feel out of control which can also result in chatter. Start with a speed control at roughly 1/2 speed, drop down to a #3 blade and work from there.
I do most of my work with FD Ultra Reverse #1, #3, and #5. Larger blades are reserved for long mostly straight cuts on the edge of the plaques I like to make.
The only fatal mistake in learning the scroll saw is fear of making mistakes.
Good luck!!!!
Bruce