A lot depends on what kind of retreat it is? Are doing the Regnum Christi Spiritual Exercises retreat?
Practical advice, wear very comfortable clothing. Clothing you can sit in, move in, and just be comfortable in. Bring a sweater or light jacket, sometimes they keep the rooms a bit cold. Bring a bag to carry materials in, journal, pens, highlighters, paperclips, sticky notes…whatever you usually need to take notes and write reflections in. (I’m a colored pen person myself.) Bring a water bottle, mints, chap-stick…
If you are staying overnight, bring enough with you to be comfortable but I would leave electronics home, laptop or tablet, and bring a spiritual book you’ve been wanting to read.
Most of all be well rested, open-minded, receptive, and eager to hear God’s message for you in this retreat.