Unless someone has done it already, and put it on the internet, you are not going to find a family tree for your family, you are going to have to research it.
There are some family trees on ancestry.com and on genesreunited.com. You could also try googling "***** family tree" and a location associated with your family.
How you start researching depends on where you are. In the UK or the US the best place to start is usually the latest published census record. I don't know what that is in the US but in the UK it is 1901 (1911 is available online but it is very expensive to view at the moment, it will be made freely available in 2011). You can view census record online but not for free. To see them for free you need to go to a library with a family history research section, or a county records office.
Of course, to start you need to know the name and location of one person who was alive when that census was taken, for which you need the help of your family. Why won't they tell you anything? They may be hiding something that happened in the past.
The other way to do it would be to order copies of your parents' birth certificates. That will tell you their parents' names and where they were from. So you can work backwards gradually like that.
Good luck with it!