A little research needed.
Hello and welcome to the RTA forums !
If you want to go on a 'Fly + drive' type break where people work out your itinerary for you it would be best done through an agent. If you want to have a very personal and memorable experience where you can go at your own pace and base the trip on your interests, it's going to take a little work to get started.
To start with you will need to do a little of your own research by searching the forums and road trip planning pages above, with a good map to hand and 'narrow' your options somewhat.
In the Northeast alone you would struggle to see all there is to see in a lifetime, but 15 days will be a nice amount of time for a trip. Look around and when you have some ideas based on your interests and goals for the trip we can help fill in the blanks and offer advice.
You will find many threads offering advice from NY, like this one that contains many links herein.