You're a new author. Nobody knows your name or your work. You've plotted, written, rewritten, beta-tested, edited, and commissioned cover art. You're now ready to load your baby up to the digital store.
You are worried. There are a million other books a reader can choose. How will your book stand out?
First of all, try to choose a category that fits your book but isn't crowded. You won't get the top 100 easily in the general romance category, for example. The book at #100 on the romance category list has a ranking of around 500 today, so the book is probably selling over a hundred copies a day. Instead, pick a sub category (gothic, paranormal, time travel), or a less crowded category (family saga), if it can be a fit for your book. This will give you a chance of getting on a top 100 list somewhere, and that means higher visibility.
Forget the tweeting and facebooking. I don't do it. I find my time is better spent in writing another book. Do make sure you have an Author Central page on Amazon, with a nice author bio. If you can add blog entries discussing your book it's a plus.
Set a low price on your book. I like the $2.99 price point. On Amazon you can gift copies of your book to friends and family. Try to gift your book to 50 people, if it is in your budget to do so, and gift them all at once. Your friends will cash in their gifts, usually within a day or two of receiving the gift email, and this will boost your rank, hopefully to levels that get you onto the "hot new releases" top 100 list in your genre, or even into the regular genre top sellers lists.
Some of your early readers will write reviews. DON'T ask for reviews. Don't even hint that you expect a shill from Mom, sister, or your best friend. A review should be given freely, and a coerced review is not only unsatisfying for the writer, but can also lead to resentment from the reader. If your book is good then the reviews and sales growth will happen organically. If you pour those nasty chemicals on the lawn it will be green today, but the pond will be polluted tomorrow.
Finally, be patient. Buzz takes time. Work on the next book.