A sentence has two main parts, a subject and a verb. There's also a third part, the verb's object. First, find the main verbs. Main verbs tend to occur directly after the subject. To find the subject, ask, "Who?", like this,
Who is not certain of the value of diagramming sentences?
Who felt compelled to include a diagram question?
Secondly, look for the verb's object. It is a word or phrase that comes directly after the verb. Please note, if the main verb is a form of "BE" (is, am, was, were, ect), the word or phrase that comes after it is called a subject complement.
Try the above, and let us know how you do, and we'll continue from there.