The word "diabetes" means "to pass" (as in "to pass urine," so we ue it to mean it causes you to pee alot).

The first symptoms of diabetes mellitus is polydypsia (drink a lot), polyuria (pee a lot), and polyphagia (eat a lot). Of course doctors wouldn’t be as highly paid if they said things like "patient eats, drink and pees a lot," so instead we say "patient presents with polydypsia, polyuria, and polyphagia."

And anytime you have polydypsia (pee a lot), you have a higher chance of being dehydrated. Especially in Diabetes Insipidus.

Now go do your homework!

You should understand the difference between diabetes mellitus (what we mean when we say "he’s diabetic") Type I (Juvenile Diabetes, autoimmune, insulin producing cells are destroyed) and Type II (adult-onset diabetes, insulin-independent, its mostly due to loss of insulin receptors, mostly due to overeating, obesity, bad diet, and lack of exercise), and Diabetes Insipidus (lots of pee, usually due to lack of ADH (the thing you release to not go pee), due to inherited disease).

So know the different types of diabetes.
And what does this insulin thing do?
Where is insulin released from, and when does the body release it?
What is a receptor?
And what happens when insulin meets (bind to) its receptor?

When you start "getting" concepts in medicine like receptors, up and down regulation, and feedback mechanisms, understanding diseases and drugs becomes really easy.
References :
biochem grad, current med student