all you get from gulanick/myers site is a link into individual nursing diagnosis pages. these pages will help you diagnose.
a care plan is the identification of your patient's nursing problems. this is unique for each patient. osteoarthritis is a medical condition so any care plan you might find is only going to be generic at best. your instructor is going to expect that you have made an attempt to find out what your patient's response to the osteoarthritis is. to do that you need to assess the patient. for nurses, assessment consists of:
a health history (review of systems)
performing a physical exam
assessing their adls (at minimum: bathing, dressing, mobility, eating, toileting, and grooming)
reviewing the pathophysiology, signs and symptoms and complications of their medical condition
reviewing the signs, symptoms and side effects of the medications they are taking
from collecting all that information you are specifically looking for what isn't normal. for instance, pain. pain is not normal. does your patient have pain in their joints? pain is a symptom of osteoarthritis. it is also a symptom of one of the nursing diagnoses.