You can publish a message to the topic as follows:

aws sns publish 
 --topic-arn arn:aws:sns:us-east-1:<account id>:my-first-sns-topic 
 --message "sending message to both mobile and email" 
 --profile admin

This will return the message ID in the console immediately. Actual delivery of the message to subscribers may take some time. 

The email message received will look as shown here. Similarly, you will also get an SMS: