If that is the photo from the top side: Bobbin thread peeking through and top stitches as a straight line usually mean top tension is too tight and/or bottom tension too loose.
If adjusting top tension doesn't change things, rethread the machine. Make sure the thread is coming smoothly off the cone or spool (and not caught on something).
If that doesn't help, take the current bobbin out of the case and try reloading it, paying attention to thread path.
If the photo is the back side, the opposite is true and bobbin tension is too tight. Check to make sure the bobbin thread pulls fairly freely. If the top tension is too loose and doesn't change, floss your tension disks - a piece of thread or lint could be holding them apart, not letting them gently squeeze the top thread.