A sender is usually the speaker during and type of communication. The speaker sends messages to the receiver during communication. While the sender begins talking, they also encode their next messages as they speak. Encoding is creating thoughts as you speak. These thoughts are then said by the speaker and decoded by the reiciever.
Assume you are in a deep conversation with your parents. You are discussing your future. Your parents ask you about the college you want to attend and you begin to encode what you will say. You say, for example, the University of Iowa. It was your turn to talk, therefor, you were the sender. You came up with the answer, the University of Iowa, making you the encoder of your message.