For climbing plants that can be grown on a trellis or arbor clematis are a great choice. You have such a wide range of colors and flower shapes from the simple single varieties to the huge doubles. The one complication with clematis, particularly for beginners, is knowing when to prune them. There are three different types each requiring individual treatment. This article by Lynne Cherot which I found on the Sensible Gardening website explains the correct way to prune your clematis.

Clematis are hard to beat for adding beauty to your garden in surprising locations. You can grow them up a wall, trellis, arbor, gazebo, fence or even a tree. They make stunning ground covers spilling over rocks, tree trunks and garden walls. From early spring straight through to late fall there are clematis suitable for growing. It?s when we have to decide when to prune clematis that the problem starts. Because not all clematis have the same growth habits, they must be treated differently. Fortunately for the gardener, clematis fall into one of three groups for clematis pruning.

How to Prune Clematis
Group A
If your clematis blooms in early spring (April and May), it belongs to this group. Group A clematis only bloom on growth produced the previous year, also called old wood. Any pruning should be done as soon as they have finished blooming, to prune later than June or to severely prune will result in fewer flowers next spring. Simply prune out weak or dead stems.
C. Montana

C. Nelly Moser

Group B
If your clematis blooms in May and June and then blooms again in early fall, it belongs to group B. Group B clematis bloom mainly on their old wood? in late spring, then produce a smaller flush of blooms on their new wood, this years growth, in September.

See more at Sensible Gardening