How can I find my whole background ancesters online for free and where?

[Shirley T]

I don't know what you mean by whole background ancestors. Genealogy is not a matter of putting a name in a websites and there it all is.

Your public library might have a subscription to Ancestry.Com you can use. What I like about Ancestry.Com is it original source records. They have all the U.S. censuses through 1930. The 1940 and later are not available to the public yet.
The have military,land, immigration and other records. They have indexes to vital records(birth,marriage and death) of many U.S. states.

Not all records are online but the ones they have will save you time and money.

Now, family trees in their website or any websites whether free or not free, must be viewed with caution. However, since this is a school project and you have limited time it probably wouldn't hurt for you to go ahead and copy anything that you find in a family tree on some of your family. The trees are usually not documented or poorly documented. You frequently will see different info on the same people from different subscribers. Then you will see the same info on the same people from different subscribers but that is no proof the information is valid as there are a lot of people copying without verifying. I found out recently me, my sister and brother-in-law are all dead. We died in Newton, Sussex County, New Jersey.
They had lots of my family on both sides of the family marrying and dying in New Jersey and my ancestry is mostly southern American colonial. It was in a tree on Ancestry.Com but if the tree had been submitted to any of the other websites it would have been on theirs also. You can make up a fictitious family tree and it will be accepted by any of the genealogy websites.
You need to understand that in case at a later time, you want to really find and come up with a good verifable family tree.

A good free source is a Family History Center at a Latter Day Saints(Mormon) Church. They have records on people all over the world, not just Mormons. In Salt Lake City, they have the world's largest genealogical collection. Their FHCs can order microfilm for you to view at a nominal fee.

I have never had them to try and convert me or send their missionaries by to ring my doorbell. I haven't heard of them doing that to anyone else that has used their resources.