Open up terminal and install jdk:

sudo apt-get install sun-java6-jdk

Use keytool to make yourself a new key:

keytool -genkey -v -keystore mykeystore.keystore -alias aliasname -keyalg RSA -validity 10000

genkey – generate the key
v – verbose mode
keystore – select name of keystore
alias – creates an alias for the key
keyalg – specifies the encryption algorithm used to generate the key. Ex: RSA, DSA
validity – when should the key expire in days? (google requires like a 50 year expiry)

The keytool will walk you through the process of choosing a password and name.  Once the key is made, you need to sign the apk with jarsigner using this key:

jarsigner -verbose -keystore mykeystore.keystore programfile.apk aliasname

keystore – keystore containing your private key
verbose – verbose mode

You will be prompted for your password. You are ready to go.