To achieve a correct swing path, place an alignment pole on your target line. Place three golf balls one to two inches away from the pole on the inside of the target line (the side that you stand on to hit your golf shots). Place one ball opposite the golf ball that you are going to hit, then one to the left and one to the right with about one inch between each ball. Now do the same on the outside of the target line. These six golf balls will create a channel for you to swing the club along.
Practice making swings with your golf club through the channel, without hitting any of the balls. Do this slowly at first and gradually build your speed up, maintain control of the club head between the golf ball and along the channel that they have created. Once you are feeling confident with this place a ball in the middle of this channel. Work on hitting this ball, without striking any of the others. If you do this correctly and swing between the six golf balls, your club head will have approached the golf ball from along the target line, will have travelled along the target line as it hit the golf ball and then extended along the target line following impact. Providing your club face is pointing at the target, the ball will fly straight at the target.
To hit straight shots this is the correct swing path to move the club head along. Work on this drill and you will definitely see an improvement in your shot accuracy.