Online is not where to start. You have your birth certificate? It has the names of both of your parents including your mother’s maiden name. How about your parents and grandparents? Do they have theirs? It should show the names of both of their parents including their mother’s maiden name. Normally death certificates will show the names of both parents of the deceased including mother’s maiden name. If you can get this information that far back you will have a good start.
Now, there was a time before birth and deaths were recorded and certificates issued. Then you have to start some real digging.
Once you have some information from birth and death certificates go to a Family History Center at a Latter Day Saints (Mormon) Church. They have records on people all over the world, not just Mormons. If you find anything in their database you would like to view and print off a copy of an original document, they can order microfilm for you to use there for about $5.. I have never had them to try and convert me nor have I heard of them doing that to anyone else that has used their resources. A lot of their volunteers are not Mormon. Use the following link to find the nearest Mormon FHC.
Have you talked to living family to find out if they have any birth, marriage and death certificates on your ancestors. Maybe they have some old family bibles, old family photos, letters from elderly and deceased relatives. A lot of this can give you clues.
Now once you have done some preliminary ground work, then use the web as a tool only. Be very distrustful of family trees on any website until you verify then information with actual documents/records. The trees are submitted by the subscribers to the websites and no way do the websites verify what their subscribers submit. It would cost a fortune. Even when you see the absolute same information on the same people from many different subscribers that doesn’t mean it is accurate as too many dingies copy without verifying. Some websites have family trees and records. Distinguish the difference. Family trees are not records but subscribers submissions. View any website that only has family trees as a trash website.