It depends on the clematis. There are several groups, all of which have different pruning needs.

Agree with Rosie

Thats all true. However, no clematis HAS to be pruned. We prune them for aesthetics. If yu dont prune your clematis it will grow...but it will look a mess, and the flowers will all be at the top of the mess. If it flowers before JUNE dont prune it unless its out of control, in which case you need to cut it back IMMEDIATELY after flowering. If it has big big flowers and flowers in May/June and again in August...you'll be safe to prune it to a good strong bud on each stem, but leave a nice framework of stems. If it flowers from July onwards you can prune it down to the first strong bud above ground level. This is a rough guide only....would be great if you could give us a name, or even a picture of your plant in flower. And dont forget to feed it...they are very hungry plants.