So if you are reading this then there is a possibility that you want to start down the road to being a Game Designer. Before we begin we should first define what a Game Designer does. Game Designers are as such they literately create the game in nearly every aspect. This includes the games rules, game play, storyline, characters and the list goes on. So we have established that Game Designers are a very (and shall I say it again very) broad term.
So you are now probably at the point where you are asking "I know all of this how do I start?" The answer start with research. Every great game has to start with an idea so the question is "What do players like to see in a game?" Trust me when I say that the only way you will know this is if you play games yourself. "Gasp did he just tell me to play games?" That is correct if you want to be a good Game Designer you have to be enthusiastic about both sides of the field. *NOTE* if you don't like the games you are playing more than likely you will not like making a game in that genre. An example of this "I do not like playing platform games very much." Further more if that is the case, how can I expect to make a platform game that will do good amongst people who do like them?
"What other things do I need to do?" Well now if you have done the above start by writing your idea down. Draw every thing you want on paper (I really do mean every thing). This includes your Graphical User Interface or GUI and what it will look like. Most importantly of all follow the KISS (Keep It Simple Stupid) principle. If you over complicate things, not only will your game be harder to make but it will be harder to play as well.
You will now need to answer "Okay I have got what I want, how do I turn it into a game?" First you will need to know what programming language you want your game in. If you are a sole Game Designer that means you will have to learn it. "Oh great I can't learn to program..." Actually you can I am self taught (still learning) in Ruby and in Java. I highly suggest getting books on your chosen language and researching what you are having trouble with online. The other route should you choose to do it is taking College level courses in the (programming) language you wish to learn.
What about game engines are they useful? Game engines are useful to a certain extent. I have used Unity before and I personally found it to be rather cumbersome (to many features instantly at my finger tips). I am currently working with RPGmaker which doesn't initially look as if it has as many features and at first seems to restrict the user to RPG only games. This is simply not true I have found it to be very versatile depending on how much programming knowledge you have (RPGmaker uses the Ruby programming language). Though when you get to a certain level in programming you will most likely want to develop your own game built from the ground up (no game engine kinda like "hey look Ma no hands").
After you near the completion of your game you will need to extensively search it for bugs. Do every thing and anything you can think of that could potentially break the game. Have your brother, sister or friends play through it over and over and over again. If you are like me at first you will find a bug every few minutes of testing. If you didn't get any bugs good for you now sit down and play it again. After you have squashed your last bug (trust me you most likely have more) you are now ready to start adding any finishing touches to your game.
This wraps up the first part of starting down the road to be a Game Designer. Good luck to you all.