Without more information, it is difficult to know what you mean by the 'seam is pulling out'? Do you mean that the stitiches are pulling out at the end of a seam? It could be either your stitch length as mentioned previously or it could be your tension.
To test the tension, sew a seam and leave a thread tail long enough to grasp with your fingers. Try pulling on ONE of the threads - top or bottom. If the thread easily pulls out, then you need to adjust your tension.