My husband and I are planning a trip from Yosemite Park to NYC. We have the whole month of March to do it. Because of the weather we decided to take a south route. What is the best itinerary and what are the must sees on the way?

Just a quick note: You just about HAVE to take the southern route since you can't cross the Sierra through Yosemite in March. Down to Bakersfield then across from there . . . I'm dashing out. More later.

Is this one way or round trip?

Where are you entering the US to go to Yosemite?
There may still be quite a bit of snow in Yosemite in early March.
How much time do you want to spend in the Southwest US before heading to the east coast?
You do not want to drive a rental car from California into New York City IMO.If you wanted a true southern route, take Interstate 10 all the way from California to Jacksonville Florida and turn the car in and ride an Amtrak train north to Washington DC. See Washington before seeing Baltimore, Philadelphia and New York arriving in each by train.

It is a one way trip. From NYC we'll come back to Brazil, where we live.
We are entering the USA from Los Angeles, where we'll stay with friends and where we'll rent a car to start our adventure.

On the southern route, you might stop in:

San Diego
Phoenix
Santa Fe
San Antonio
Austin
New Orleans

Then head north and stop in:

Savannah
Charleston
Washington DC
Philadelphia
New York

(and plenty in between!)

Do you like natural beauty or are you more into cities?

There are lots of National Parks and State parks that are really wonderful for natural beauty, hiking, history etc. To find the NPs go to www.nps.gov, for the state parks go to each state's web page and click on parks.

For cities, go the web page for the city to find, museums, theater, concerts, ball games etc.

I wouldn't be too afraid of heading along any of the cross country roads south of I 70 and you will probably want to do a bit of zig-zag in any case.

Since Tioga Pass is closed you can either take the interstate through our Central Valley, or drive to San Francisco for a few days then drive Highway 1 along the coast to LA with at least two overnight stops. Then get on boring I-10 to drive the southern route.
Hopefully you have reservations for Yosemite as places book up a year in advance.