If you're driving, it takes about an hour. Of course, you're in or near New York City so traffic can be a major factor. There are several buses and vans that will take you between the two--just go to the public transportation section of the airport and there will be desks there for you. The average price of these is around $25. You will usually have to wait until there is a van-load of people before they'll take off which can be frustrating but if you're there at a decent time there is usually plenty of people looking to make that trip as well. Or you can always take a cab. This is the most expensive option, but also the fastest.

Another option for you, Mary, is to take one of the private shuttle services between EWR and JFK, though this may depend on your arrival times as shuttle service typically only run between 7:30 a.m to 9:30 p.m. on weekdays. Trip duration should be a little over an hour and costs around $30 one-way (don't forget the tip, too!), so obviously more pricey than the train option. But faster. Taxi or limousine service is one other option, though this is obviously your most expensive choice. The Port Authority of NJ & NY lists more airport connection options here: http://www.panynj.gov/airports/jfk-airport-connections.html

It's a pain but doable: Take the A/C subway train from JFK to Penn Station and then the New Jersey Transit train to Newark. The subway portion will take at least an hour if not more. Penn station to Newark is about 30 minutes. Minimum two hour trip, I'd give yourself 3 or more to be safe.

If more than one person. consider renting a car.

(note: Newark Airport stop, not the Newark stop, there are two)