Start with what you know: the names, birth/death dates and places of your parents and grandparents, if you have them. If you still have living relatives, they'll be one of your best sources of information. From there, choose an online genealogy platform that allows you to create a family tree and start entering the details you get. (Even guesses or approximations are okay when you're getting started.) Then, you'll use online databases to find documents that support those facts, like census records or birth certificates, if you don't already have them in your possession.