Choose the subject of the sentence and determine whether it is singular or plural. If the subject is a pronoun, determine whether it is first, second or third person.
Choose a linking verb that agrees with the subject in number. If the subject is singular, use a singular linking verb such as "is" rather than the plural linking verb "are."
Select a linking verb that agrees with a subject pronoun in number but also in person. "Am" and "is" are both singular and would agree with a singular pronoun, but "am" is first person, as in "I am," and "is" is third person, as in "He is." The more confusing choice here comes when you use the second person pronoun "you." Whether used as a singular or plural pronoun, you always take the linking verb "are."
Use a conjugation chart, such as the one found on Tips for Teaching Grammar or one found in a grammar handbook, that you can rely on for forms of the verb "to be." This will simplify the process of finding the linking verb that agrees with the subject until you have used the chart enough that it is committed to memory.