The official place to sell Android applications is the Android Market. You can view the market by visiting the website here: https://market.android.com/. Reading the market can help you see which apps are in demand and which area you should develop an app for. All apps need to be digitally signed before they can be sold in the Android Marketplace.
The Distributable File
Android applications are packaged as APK files. You can use the ADT plugin inside Eclipse to digitally sign and compile your applications, that is what we will be focusing on here. Digitally signed applications have a public and private key. A keystore is a place where your (personal) certificates are held. You never want to lose your keystore, make a backup and put it somewhere safe.
Create an APK File
Right click the project and select Android Tools then Export Signed Application Package.
Next, give a name to your project
In the next window you will click the Create new keystore radio button. Select a location on your hardrive by clicking the Browse button and save APK your file, you can name it anything you want. You will need to supply a password by typing it in the two boxes below, like seen below.
In the next part you will need to provide some info, we will only provide the very basics. Provide the password and your name. I am not sure why you must provide at least a 25 year certificate expiration date but... yea, 25 years..
You can sign up and get your Android market account from here, http://market.android.com/publish