Before you design your own house, you have to remember to take into account your budget and your lifestyle. Putting things down on paper is fun and you can get creative but you can't get carried away.
When you are deciding what type of house you want to design, you need to ask all members of the family and try to get input from everyone. Narrow down the ideas to ones that are the most practical and affordable. Yes, it would be nice to design that 5 bedroom house with a 3 car garage but do you really need all that and can you afford it?
A good place to start is with a design book you can get inexpensively. This will give you some ideas to start with. It is easy to go overboard and choose a plan that looks great on paper, but does not suit your family. A house plan design book will help you get started.
Once you have more of an idea of what you want, you can use an online tool, hire an architect, or a home designer to help design a house for you. You might like to start with an inexpensive online tool and then take your design to a professional who will be able to make the necessary modifications. If you design your own house first, you will always have the satisfaction that it is "your" house and you were the one who came up with the plans.