Focusing is one of the harder parts of shooting astro. Put it in manual focus, use the back screen to focus and zoom in on the focal point if your camera supports it. Pick a bright star and adjust your focus until it's as small as possible.
Also keep in mind the rule of 500 to get sharp shots. You don't want your shutter speed to exceed 500 / 35mm equiv. focal length, i.e. if you're shooting with a 12mm lens on a crop sensor, 500/18 = ~28 seconds max exposure to keep things sharp. If you go too long you'll get a little bit of star movement. Shoot wide open at whatever exposure time you calculate, then play around with your ISO until the exposure looks right.