As @Laurence Dawson mentioned one way to do this is just by opening your project in eclipse then
Right click on the project > Android Tools > Export Signed Application Package
BUT you MUST create a Keystore prior to do this, this is really easy and simple the option will appear after you select "Export Signed Application Package" from right click options, it will ask you to "sign" your APK (in order to Beta/Alpha/Production publish) this process is done simply by creating this Keystore, once its created just select this new keystore (it will appear right in the options of "Export Signed Application Package") and voila! then it will just ask you for enter the path where to upload your signed APK ready for google developer website!