Designing a computer program, in layman's terms?

The first thing to consider is inputs and outputs. What do you want to put into this program and what do you want to get out?

What do you want to store?

A database is not a program. A database is where you store data. So what data do you want to store?

If it is a family tree then pick a name in the middle of the tree as an example.

There will be a first name, middle names? family name. Parents, children? marital status, wife (There could in theory be more than one wife). Marriage date. Date of birth, Date of Death.

This information you are storing is not organised as yet but put as much information down as you want to store. Someone can then organise it for you.

Have fun.

The FIRST thing you do is something called requirements gathering. That means, you need to figure out exactly what it's supposed to do, what constraints (necessary concurrent users, interfaced software, time limits, etc.) there are, and who your target audience is.

Next decide what libraries and tools you'll end up using and where your source code will be maintained.

Next is to use paper to draw what you want the interface to look like. This is called prototyping.

Next figure out how to lay out the SQL for your database.

Then, built it. This is an extremely rough set of steps for designing an application. If you really want more information, you probably want to look at books or websites that talk about the phases of the software design process.