The FamilySearch Blog has a great article that very simply explains how to find ancestors using the Family Tree on FamilySearch:

FamilySearch Family Tree is the world’s largest shared family tree. This free, public tool strives to have one public profile for every deceased person who has ever lived. You can search for information and find your ancestors here, even if you’ve never visited FamilySearch or connected yourself to the Family Tree.

Gather what you know about your deceased ancestor—such as his or her name, birth or death information, and perhaps the name of a parent, spouse or child—and follow the steps below to see if he or she is in the Family Tree.

Find Relatives Using the Tree Search

If you want to search for a relative who may be in the Tree (even if you haven’t used the Family Tree before) you can go to FamilySearch, and under the Search tab, choose Family Tree. From here, you’ll either need to log in or create a free FamilySearch Account.

Read the rest of the article to see the rest of the steps and also to learn about the following:

• Tips for Finding the Right Person
• Finding and Adding Relatives from Your Tree View
• Searching for Unknown Relatives in Records