The lever on the top front is the ZZ width (stitch width) adjuster. Move it to 0 for straight stitch.
The round knob to the left is the lock to the adjuster lever. Loosen it for the adjuster to function.
That being said, take the top off, the face plate off, tip it over to get to the bottom and oil every metal thing that moves or touches another metal thing. Grease the gears with Tri-Flow or Singer grease.
Make sure to check the feed dog raise / lower controls to make sure they are free to move.
Tri-Flow is the best oil to use to free up a sticky machine. Clean it as good as you can first, I use denatured alcohol on the insides, the oil oil oil oil it till all of moves freely. Then wipe it down and run it really good for a while to distribute the oil.
Joe