Gramps is a great open source tool to create, maintain and visualize your family tree. I had a great time asking around my family about our ancestors and exporting beautiful ancestor tree diagrams and descendant tree diagrams.
$ sudo apt install gramps
.debfile from here and install it:
$ sudo dpkg -i gramps_you_downloaded.deb
When you first open it, it asks you to create a family tree. This is the document inside which you can maintain one family tree. Give it any name you wish.
On the left are different views into your family tree. Out of these, I found that I can get by using just People, Relationships and Charts tabs.
Add a person: Start off by adding entry for one person (say your earliest ancestor or yourself) by going to the People tab and then choosing + (Add a new person) option in the topbar. You can enter Given Name, Surname and in the Events section at the bottom, add a Birthday. Get used to this dialog, you will be seeing it a lot when adding or editing people to your tree.
Add a relationship: Click a person's entry in the People tab and then switch to the Relationship tab to add partners (spouse) and children. Choose Edit → Add a Partner to add a spouse and their details. At the bottom of that dialog, you can add one or more Child entries.
Charts: Switch to Charts view to see ancestor tree of a chosen person. The default side view is quite confusing to me. I like to change the chart view by choosing View → Configure and choosing Vertical ↑ as the Tree Direction. This ancestor tree view looks more intuitive to me. You can double-click any unfilled ancestor box to add that person to the tree.
Ancestor tree: A beautiful ancestor tree for any person (say you or your child) can be exported to PDF. Select the person in any view and choose Reports → Graphical Reports → Ancestor Tree. Note that there is an option in there to resize the PDF to fit the entire tree. You can later convert the PDF to PNG using ImageMagick tools, like described here.
Descendent tree: A beautiful descendant tree for any person (say your oldest ancestor) can be exported to PDF. Select the person in any view and choose Reports → Graphical Reports → Descendant Tree.
Tried with: Gramps 4.2.6 and Ubuntu 16.04