I get a lot of questions about signing in to FamilySearch.org. In order to sign in, you must first register. It is not necessary to sign in at all to view most of the resources on FamilySearch.org. Family Tree is one exception. You must register and sign in to even see the link to Family Tree in your browser. There is also a major ambiguity on the FamilySearch.org start up page. There are two "Tree" links. Here is a screen shot showing the two different links:
The link entitled "Trees" goes to a search screen to search primarily the Ancestral File and the International Genealogical Index (IGI). You will not find anything on Family Tree using this link.
The second link is entitled Family Tree and it only appears after you register, sign in and are linked to Family Tree. Clicking on that link takes you to the Family Tree program.
Registering is a rather simple process but to do so efficiently, you need some basic information. If you are a member of The Church of Jesus Christ of Latter-day Saints (the Church), you need your membership number and your birth date to register for an LDS Account. You will also need an active email address. You can register without an email address, but you will not be able to collaborate or communicate with other users of Family Tree without an email address. All other users, can register for a FamilySearch Account with a name and an email address.
The only difference in viewing the website between members and those who do not register as members, is that LDS Temple ordinance information is not visible to those who are not members.
As a side note, there are a few collections in the FamilySearch.org Historical Record Collections that require you to sign in to view the records. This is due to limitations placed on the records by the original repositories and not a requirement from FamilySearch per se.
It is important to understand that no one owns their genealogical data. If you post data to the Internet, anyone can use that data with impunity. Do not post personal family information you do not wish to share with the world.
Once you are registered, you do not automatically see the Family Tree link. You have to be connected to Family Tree. Some users have experienced difficulty in viewing Family Tree. You may wish to read the comments to my prior posts on this subject for some discussion of the problems encountered. Please feel free to contact FamilySearch through the Help menu for assistance. It appears to me that most of the viewing problems stem from not completing the registration process, especially setting your preferences.
Here is the link to the document explaining how to link to Family Tree. The document is at the bottom of the page and is called Gaining Access to the FamilySearch Family Tree (pdf).