Can't get a better breakdown than the source.
Easy Mode:
Since you're using eclipse, just click "File->Export->Export Android Application". Follow the wizard's instructions. You'll need to create a keystore with password. Within the keystore, you'll need to create a key to sign the app with. The key can have the same password as the keystore or a different one. KEEP THE KEYSTORE AND PASSWORDS. You will need to use the SAME EXACT KEY every time you want to update your app. Failing to do so will require you to resubmit your app as a different program on the store.
Go to https://play.google.com/apps/publish/. Create your account. You'll have to pay a one-time fee of $25 to submit. Once there, you'll have access to your console. Click the button "Upload Application", follow the instructions, input the info, click "Save". If everything is the way you like it, click "Publish". Grab some coffee, read a book, play a game, go to the gym, whatever. Come back a couple hours later and you and everyone else will be able to download the app from the store.
This is all assuming you have ONE apk that is LESS THAN 50MB in size. Things get more complicated when you have to implement expansion files, multiple APKs, app licensing, or any of the other Google Services.
UPDATE
As of September 2015, the maximum size of an APK on the Google Play store can be 100MB before you have to use expansion files.
