After developing Android application you want to release it. To do that you have to prepare apk package. One of the steps is signing apk file. This can be done using Eclipse ADT plugin export wizard or jarsigner and keytool from standard Java distribution. I want to show you how to sing apk file using Eclipse.
Right click on your application in Package Explorer and select Android Tools -> Export Signed Application Package...
You will get this window :
Just specify project name here. Click Next.
In next window you can choose keystore. If you exporting your application for first time choose Create new keystore, select name for keystore and your password. Remember that password! It is very important because you won’t be able to update your application if you loose you key or forget password. Google check if your update is signed with same key that was used to sign earlier release. If not user won't be able to update and the only way to upload your application to Android market is change package name and release it as a new product.
Next step is providing details about your key. You should use one key to sign all your applications. Just fill this window with information about you. Remember to choose long validity (for exemple 30 years). After you finish click next.
Here you have to choose destination of apk file.
When you click finish you will get apk file which is ready to upload to Android market.