You can get great star shots with the kit 18-55mm lens (I don't recommend trying astrophotography with any zoom lenses; zoom lenses at night are mostly useful for shooting the moon, but otherwise won't serve any purpose). Of course, because this is the kit lens, it's not going to be the absolute greatest. Aperture (or f-stop) is important. Your aperture is a device in the lens that controls how much light the lens takes in. A higher number means a smaller aperture diameter, which means the lens takes in less light. A lower number means a larger aperture, which means the lens takes in much more light. Finding a lens with the largest aperture possible will, of course, allow for your lens to take in more light and get better star shots. However, lenses with these large apertures are pretty expensive, so if you're on a tight budget or just not sure if you want to spring for a fancy lens just yet, your 18-55mm will do just fine.
Arguably, the most important part of astrophotography is location. If you try to capture stars in the middle of a city, you will be disappointed. Even the lights of the average neighborhood will greatly reduce just how many stars you see in the night sky. Light pollution will show up in your images as well and can force you to step down how much light you allow the camera to take in, which unfortunately also means you don't get the greatest star shots.
http://www.jshine.net/astronomy/dark_sky/ is my favorite tool for finding light pollution.
Next, we focus on... well, focus. Set the camera to manual focus. There should be a switch either on the camera body or the lens. Switch from AF to MF. Find a distant light or a star and focus on it. Take some test shots and zoom in to the stars on your LCD display to see if they are sharp. If they're fuzzy, adjust your focus.
Now for the settings. Your camera's shooting mode should be manual, which is the "M" on the dial on the top of the camera. Make your aperture number as low as possible (i.e. the largest aperture you can get with that lens). Maxed out at 18mm, I believe that means your lens's maximum aperture is f/3.5. I shoot with a Nikon, so I don't actually know the layout of the A55 beyond a quick search of what the camera looks like, but I believe you adjust the aperture on the A55 by holding down the "AV" button (on the top of the camera next to the movie button) and moving the control dial (on the front of the camera next to the shutter release button). But don't quote me on that; you may just have to hit the "AV" button and a menu will pop up. You'll want your ISO set to 800 or more. The higher your ISO is, the noisier/grainier your images will be. This is set with the "ISO" button on the bottom of the "wheel" to the right of the LCD. Finally, you'll want a long shutter speed. Now this is determined based on your focal length. Typically, you'll want as wide an angle as possible, and that would be 18mm with your lens. We use the "rule of 400" to calculate the shutter speed. Basically, you divide 400 by your focal length, which in this case would be 18mm. 400/18=22 seconds. This is a general rule in order to reduce the blur you get from the movement of stars (well, technically, the rotation of the Earth). So you'll want your shutter speed to be 22 seconds or under in order to make your stars little dots instead of little streaks. If you move the control dial (front of the camera; no need to hold anything down this time), this should control your shutter speed.
As Captain-Heather mentioned, a remote release is also important. However, if you don't have one of these, try setting your camera to a delay. That way, it has time to steady itself after you hit the shutter button.
And that's just a quick crash course into astrophotography and settings on the A55! Let me know if you have any other questions!
