安卓运行项目报错，我只是个新手求解？？

08-09 09:44:26.361: D/AndroidRuntime(404): Shutting down VM
08-09 09:44:26.372: W/dalvikvm(404): threadid=1: thread exiting with uncaught exception (group=0x40015560)
08-09 09:44:26.382: E/AndroidRuntime(404): FATAL EXCEPTION: main
08-09 09:44:26.382: E/AndroidRuntime(404): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lp.joke/com.lp.joke.MainActivity}: java.lang.NullPointerException
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.os.Handler.dispatchMessage(Handler.java:99)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.os.Looper.loop(Looper.java:123)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread.main(ActivityThread.java:3683)
08-09 09:44:26.382: E/AndroidRuntime(404):  at java.lang.reflect.Method.invokeNative(Native Method)
08-09 09:44:26.382: E/AndroidRuntime(404):  at java.lang.reflect.Method.invoke(Method.java:507)
08-09 09:44:26.382: E/AndroidRuntime(404):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-09 09:44:26.382: E/AndroidRuntime(404):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
08-09 09:44:26.382: E/AndroidRuntime(404):  at dalvik.system.NativeStart.main(Native Method)
08-09 09:44:26.382: E/AndroidRuntime(404): Caused by: java.lang.NullPointerException
08-09 09:44:26.382: E/AndroidRuntime(404):  at com.lp.joke.MainActivity.onCreate(MainActivity.java:20)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-09 09:44:26.382: E/AndroidRuntime(404):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
08-09 09:44:26.382: E/AndroidRuntime(404):  ... 11 more
Android