If you are learning to sew for the first time, I would recommend this technique.  Take a piece of fabric, preferably a cotton, calico or some other cheap fabric.  Draw lines using a ruler and a pen or pencil.  Make grids like norts and crosses.  Now sew along these lines until you can sew a straight line without moving off to the side.

 Now draw two lines side by side, about 2 mm (1/4") apart.  Now sew a straight line on the line on the right hand side, and take notice where the sewing machine foot is in comparison to the line on the left.  Don't look at the stitch you are sewing.  Try to line up the line on the left with a point on the sewing machine foot, and watch this rather than your stitching.

Practice this until you can sew without looking at your stitching, but at where you can position the sewing machine foot.  When you master this you will nearly always sew a straight line.

Happy sewing

Judith aka genie