Unless you want to look like a rank amateur in the bar or restaurant where you work, you're going to need to know how to use a serving tray. For glasses or small dessert plates, you'll typically use a small, round tray, which can still be tricky if not properly balanced. For larger plates, use a larger, oval-shaped tray, which requires an even more carefully-balanced maneuver.
Loading the Tray
The general rule is to load the heaviest items near the middle of the tray and lighter items around the edges. If you're loading up tall drink glasses, though, place them in the middle or they could topple when you move the tray even slightly. If all of your plates are about the same weight, place them on the tray in the order people are seated at the table you're serving so it's easier to remember who gets what.
And Carrying It
Place a flat hand under the center of the tray and allow the tips of every finger, as well as your palm, to rest on the tray surface. If the tray is heavy, bend at the knees and tuck your elbow in closer to your body to stand and raise the tray up. Rest one side of the tray on your shoulder as you stand up to help you balance it. Grab a tray stand with the other hand. Walk carefully to the table you're serving, taking the route that offers the clearest path, and then open the tray stand, grab the tray with your now-free hand, and set it on the stand.