Why does my sewing machine jam and the thread breaks?

I just got this Singer stylist 534 its an old one but it is my first sewing machine underneath the plate the thread will just jam and look all ugly i tinkered with it for a while and then on the top the stitches look regular but underneath the stiches looked jumbled and crappy. I dont know if the machine is just broken and how much would it cost to fix it should i just buy a nice new one. I got this one for only $20 (took a chance i guess) but the home and people looked like they took care of it. i was going to buy a manual for it but should i spend $15 on a manual or just buy a new one? I got this one just to learn and practice and get the hang of it. should I go and have a place look at it? Any suggestions would help thanks!