You don't say if it's a Floribunda or an old fashioned Rambler. The main difference is the Floribunda blooms most of the growing season whereas the Rambler blooms only once and then on "old wood" (last years growth.
If it blooms only once, you can prune it right after the last blooms fade. It can be pruned heavily and won't hurt it. Be sure to prune out a lot of the center to give it better air circulation and prune away any old, derad or broken stems (be sure you have some sturdy leather gloves and a sharp by-pass pruning tool).
If it's a Floribunda, then don't prune it until early next spring when it's still dormant. Again, prune the center for air and cut away dead wood. You can decide on the shape you'd like...
Best of luck!
(There are several other varieties, but yours sound suscpiciously like a Rambler...)