As a Contractor, I have found that it's best to start out looking at a ton of plans that are already in existence. Eplans.com is a great place to start. Put in your dimensions, number of bathrooms, bedrooms and whatever else you have to have, and then look at every plan carefully. Then if you find something you like, you can either just copy that plan, or modify it to make it more tailored to what you want.
I personally use Chief Architecture for drawing plans. It's simple to use and you can add furniture to it to see how everything fits and flows together. What I find happens with beginners is the fail to plan on how the rooms will be furnished, and how you will get furniture moved into the space. They also forget about things like the water heater, HVAC system and Laundry. Will there be a garage or covered parking area, and how will you get groceries into the kitchen? Then there are those who have hallways going everywhere, which are wasted space, but necessary in some applications, and not in others. With the door to the toilet be exposed to the living room? Will the kitchen sink have a view or be up against a wall? how will you vent the stove? With Chief, I take all of that into account by placing everything that is supposed to be in the house, where it will go. Surprisingly, most of the time, they don't fit on small homes!!!!