Fascinating! I started out with FamilySearch.org, and whatever free family-tree maker they promote. They really have a huge amount of stuff available online, for free. I browsed through a ton of other sites, both professional and personal, just googling the names of key ancestors to see what turned up.
Then it turned out my uncle had an Ancestry.com subscription but hadn't done much with it, so he gave me the password and asked me to work on it--that's what I use exclusively now. It took me a while to figure out how to use it, but now I'm used to it and I really like it, and they have a lot of records. I also like that I can "invite" non-members to view the tree in its entirety, for free, so I can share it with my family if they ask for it. I back up the gedcom file often and I make sure to save the pictures and stories onto my own hard drive as well, just in case.
Ancestry.com often links to records at Find-A-Grave, and I've found that to be incredibly useful, even moving, for some branches of my family where the cemetery is well-covered. It's remarkable to go online and find an ancestor's grave photographed, and see a message from their great-granddaughter or something posted there.