App installed on device but doesent show up

Hi,Im new to android development and wrote a simple code to run a simple application.But when i run it on the emulator or on my device,the console window says that the app has successfully installed but nothing opens up on my emulator or phone.can u plz suggest a possible way of solving this problem.

Manifest File

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://ift.tt/nIICcg"
package="com.example.firstapp"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />
<intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/title_activity_main" >
    </activity>
    <activity
        android:name=".First"
        android:label="@string/title_activity_first" >
    </activity>
</application>

</manifest>