Got a sewing machine, got it threaded, and now I'm Sewing Machines Parts and Sales in Los Angeles

I bought a singer from goodwill upon recommendations here. I want to alter some shirts. I get the thing threaded up and when I push down on the pedal it starts to sew then seems to get caught up and knotted up underneath..not sure what I am doing wrong.

EDIT: I’ve read maybe the tension is wrong? IDK what to do I’ve just been trying to teach myself to sew a straight line on an old shirt, once or twice I have gotten it to work but usually it just ends up staying in one spot and getting knotted up.