Flowergirl your clematis has been instore and need to harden off before it can be planted, by being put outside in the day time (but not when it is very cold) and either taken indoors or put in a frost-free greenhouse in the nights. Personally, I don't like planting very young plants straight into the ground and would prefer to pot them on to grow on for another year into potting compost where they gind it easier to make plenty of root. Don;t let the top grow taller than about twelve inches, so that it can put its energy into making roots and also so that it produces multiple stems. If you do plant it into the ground, after hardening it off, dig a hole at least twice the size of the pot, sprinkle bonemeal into the bottom of the hole and fill around the roots with good soil. Plant the clematis (whether or not it is into a pot or the ground) with the top couple of inches of stem under the soil.
It might be an idea to tap the clematis out of its pot and have a look at the roots. If they are pretty small and do not fill the pot, the clematis can grow on in the existing pot for a while. Remember not to let the plant grow long stems or to flower and remember that clematis like their roots in shade and their tops in some sun. For the first two years, they need to concentrate on making a strong root system. Garden centres let them grow and make flowers because this will make them easier to sell, but by cutting them back fairly hard and sacrificing the flowers, you are making an investment in their future.
