As tom said lilacs don't need much pruning, take out any dead branches and take out branches that are rubbing across others.
If for some reason you do want to prune it, now is the wrong time. It has already set the buds for next year's flowers and you will be pruning those off. If you want to prune, do it right after it has finished blooming.
Tell us why you want to prune it? If it is too big or too tall, what you want to do is not cut any branches back, but cut some (up to 1/3 of them for a healthy tree) off right at ground level. Do that again for two more years and you will have a rejuvenated and smaller tree.