The first time you go to "My Tree" you will be prompted to add your immediate family members.

After the first node has been added. Hover over a node and click the "+" that appears on the bottom of the node.

If you do not see the + icon, it is because you don't have permissions to add relationships to that node. This might be because the node may belong to another user.

Each person can only have 2 parents. If you want to add a step-parent, add them as a spouse to one of your parents.