Thanks for your question.  While pruning lilacs can indeed make them bushier, your plants are still quite young.  It might be advisable to wait a year.  Next year plan to prune right after they bloom.  If they don't bloom, time it for about the end of May or when you see other lilacs in your area fading.  Make sure they get plenty of water this summer, whenever the top inch of soil feels dry.  You might also consider gently staking some of the canes until the plant grows more.  This could be important in the winter so the canes don't get bent down should there be snow.  In addition, you might consider getting a soil test done to make sure your pH and fertility are appropriate.  Penn State offers one for $10.00.  You can download the form and instructions from https://agsci.psu.edu/aasl/soil-testing/fertility/soil-fertility-submission-forms.  Be sure to follow the sampling directions carefully, as a good sample is critical to meaningful results. 
I have attached a Fact Sheet that provides some additional information.  Please let me know if you have any questions.