What type of sewing thread are you using? How is it wound on the spool? Check to see that it is coming off the spool in the right direction to the machine.
Make sure you are catching the tension disc when you are threading the machine. It can be easy to mis-thread.
Raise the presser foot all the way before you start to thread the machine. ( I don't know your machine but this is one tip I use for my machines.)
Is the bobbin wound correctly? Make sure the thread is wound firm in the bobbin and not squishy.
Is the bobbin inserted in the correct direction for your machine? Make sure the thread is coming off the bobbin the right direction.
What needles are you using and type?
Check the dial setting to make sure you are on the straight stitch and not a zig-zag.
Good luck!