Read the sentence out loud. Hearing the words will help you become familiar with the sentence.
Look for the verb. Verbs come in two types: action words, such as "run," and verbs of being, such as "is," "was" and "are."
Consider the sentence, "Jake runs down the block." The word "runs" is an action verb. The word conveys what is happening in the sentence.
Ask yourself, "Who is completing the action of the verb?" In this case, ask, "Who runs?" Since Jake runs, Jake is the subject of the sentence.
Take the sentence "Fido is my dog." "Is" is the verb, but it's a verb of being. Ask yourself, "Who is?" In this case, it's Fido. The sentence is describing Fido, so Fido is the subject.
Read this sentence. The verb is "read." Who is reading? It's you. With sentences that give commands, the subject is "you."