should i prune in first year plants have only been planted about a month and have finished flowering

Don't know how tall the plants are, but yes, you should prune now to encourage shoots from low down so you don't end up with a hedge with bare wood at the bottom and green at the top. Usual advice is cut down by two thirds, but as I said, not sure how tall your plants are, so you could play safe and take half the height, assuming they're over 18 inches.

Give it a light trim all over to start off the 'hedge' shape. Remember to get a slope on the sides so that light can get to the bottom part or it could become bare around the legs. It should shoot well after a light prune.