If you chose to move onto another idea in the previous step, as many new writers will, it will make this step a lot easier as you can learn from the mistakes you made from your previous attempt. However, even if you managed to complete an entire book, as hard as it is to admit to yourself, unless you’re really good, your first book will probably be average.
Simply by the practice makes perfect rule, your next project will inevitably be superior to your second. For this reason, your third idea is going to be the idea that you’ll want to make a little gimmicky… or unique at least. This is because this is the one you’re going to be using for the next step. However, unless you’re really lucky, it’s highly unlikely that your first project is going to be picked up by professionals.
My seventh piece of advice: stick with it. The more you write, the better you’ll get, so your later projects will inevitably have a better chance at being published. As heart breaking as it will be to admit that your earlier works won’t be published, when you finally do publish something you will realize that writing them wasn’t a waste of your time because of what you learned from the experience. If the nostalgia is still there, you can always rewrite your older works when you have more experience to do them justice.
I’d written five completed stories before I got picked up. Of those five, I tried to sell four, of those four, I realized only three were really sellable, of those three, I’ve signed two contracts and of those two, one is being published. In order words, unless you’re really good or really lucky, it should take you around 4-5 complete stories before you actually publish anything.