When you say "not sewing well" do you mean the thread is shredding or breaking or are there bird's nests of thread underneath? Or is it not feeding the fabric?
You said you have changed the needle. Did you put it with the flat side of the needle towards the left? When you changed the bobbin, did it follow the correct path? Also when you rethread the top thread, make sure not to miss any steps. I've had thread go wonky on me because the top thread has jumped out of a spot.
I use my Singer 15-91 all the time. It is a great mechanical machine and I have learned that most problems are solved very easily and just take a bit of thinking. It like a good oil, like every eight hours of sewing.