Sign a public PGP Key

I have just installed the PGP command line package. I have a public key from a outside enttiy that I need to use to encrypt a file. 

I have successfully imported the key, but shows as invalid and I have been unable to get the key recongized as valid. 

I tried to sign the key but I keep getting error 1052 - No signing key specified. 

I am totally new to the product - forgive my ignorance. 

But what do I need to do to get this key to be recognized as valid and then use it to encrypt a file?