My dog has a red, irritated spot on her neck. What should I do?

That lesion is very irritated on Nana's poor little neck! Lesions like this can be related to trauma, a skin infection, the beginning of a hot spot or an infected cyst under the skin. I can't tell from the photo, but if it looks like bruising, then there is also a concern for a bleeding disorder and she needs to get to the vet right away.
Assuming there is no bruising, I recommend to have the lesion examined by the veterinarian since it is so red. There are a few things you can do at home that may help until you get her into the vet. You can try adding in an omega 3 fatty acid supplement for dogs, this can help improve the skin health. You can wash the lesion with soap once a day and rinse well (don't wash her whole body, just the spot). Protect the areas from licking or scratching. A cool (not cold) compress twice a day may also make it feel better. Best of luck and please feel free to post any additional questions.