It is entirely up to you how to start writing. There is no magical moment that tells you how to do it, or when to do it. You just do! You wake up and decide, today is the day, and begin your commitment to your piece.
Start by finding someplace comfortable, without any distractions where you can sit for an extended period of time and can really let your mind clear itself. It is important to not have any worries stored in the back of your mind- don't worry about your chores, your kids, what to make for dinner for just a little while. Foreign thoughts will be your biggest distraction. Instead, settle in and think about what you want to accomplish in your writing. Is it a letter to someone? An essay for school? A story? If so, is it a short story? A poem? A novel?
Once you have picked a goal, don't stop. Put your pen to paper or fingers to keyboard and write. It doesn't matter if your spelling is bad, if your grammar isn't perfected. You can always go back and edit. Just focus on getting everything out of your head and onto paper. It is best to see visually what you have to work with in front of you rather than keep it stored inside of your memory, where it can get lost perhaps permanently.
If this is your first attempt at any sort of writing project, don't put it off due to fear of failure. No first draft comes out perfectly. Writing is an art you have to practice, like any other form, be it a musical instrument or pencil sketches. You just have to decide once and for all that now is the time to start.
Think of whom you are speaking to in your writing, your target audience. Will it be to someone you love? Small children? Maybe an unknown audience of your peers? The easiest way to go about with your diction is to write it exactly as if you were speaking to them. Don't try to use big words out of the thesaurus that you have never used before. Rather, use the words you know already and can wield well. They are your tools. Use them to express exactly what you are trying to say.
Don't stop after your first go. If you are not finished from the first sitting, come back. Once a day for an hour, once a week, just keep going! Make it a personal goal to finish your creation. Make it fun, but not a chore. Like with everything you do, writing takes hard work. The more you put into it, the better it will be when finished. It takes authors many years to perfect a piece of writing. But they once were where you are, and they too once had to decide to take the leap and decide how to start writing once they had a great idea. Maybe one day you will be on a bookshelf too!