Is there one central stalk, or are there several shoots arising from the crown? Is there new growth sprouting from the crown? Has the plant lost all its lower leaves? Are there any sprouts along the main stem?
When pruning, always cut back to just above a node - the area where a leaf or branch meets a stem. If you look closely you may be able to see a small bud, called an axillary bud. If the leaves have fallen and the stem is bare, look for the leaf scar that indicates where a leaf was attached at a node, and make the cut above that. Nodes have latent buds that will sprout once the main shoot is removed. It's best to leave some foliage on a plant so it can continue to photosynthesize and manufacture food for growth.
Some plants will resprout even if you cut the entire plant back to its base; however, removing all the foliage is stressful to a plant. If you have just one central stem, you may need to resort to such severe pruning and hope the plant is strong enough to recover.
A good place to begin is to prune back half of the longest branches back to about a third their length. If these branches have side shoots even further down toward their base, you can prune all the way back to those shoots. For example, if a geranium has four main branches with lots of sprouts, we'll prune two of the branches back hard, almost to their bases, and we'll make less drastic cuts on the remaining stems, leaving some foliage. (A side benefit of pruning is that you can root the cuttings!)
Since drastic pruning stresses a plant, place it in a bright spot out of direct sun for a week or so, then return it to its former home.
Fertilize the plant with a soluble, all-purpose fertilizer diluted as instructed on the label. Check to see if the plant needs repotting. If roots are growing out of the drainage holes, or if you remove the plant from its container and see mostly roots with little soil, it's time to repot. Remember, since you have reduced the amount of foliage on the plant, it will probably require less water than before pruning, so adjust accordingly.
You should see new growth within a week or two. Once new shoots have two sets of leaves, pinch out growing tips to encourage branching, and continue doing this on subsequent branches.