are you using a high loft batting? something like warm & natural which allows up to 10" between quilting lines is ideal to learn with-
some of the batts require 2" to 4" quilting (quite dense) and can be part of the problem- but too much loft can also be a problem- lofty batts are better used for tied quilts.
check the packaging and see what it says is recommended- try to lengthen your stitch length- often a longer stitch will keep your lines from puckering.
and slow down...trying to hurry the fabric through faster than the walking foot/feed dogs are going will also cause these problems