Thanks for writing. There are a few different ways to prune lilacs, depending on what you're going for. If you have the true tree form, it may not need any pruning, unless it's getting in the way of something (growing into a house/garage, etc.) or it's do dense there are branches growing into each other. In this kind of case, you'll want to remove the offending branches.
If you have the shrub kind and it's grown huge, then you can renovate it by removing a couple of the oldest stems this year. Next year, remove a couple of the next oldest stems to encourage it to produce healthy, vigorous new stems.
---Justin, Senior Garden Editor, BHG.com