The term security has such wide-reaching implications that it is often quite hard to define in one single sentence. There are a host of different types of security, all of which aim to either validate, protect, or authenticate actions taken on or over your server and ensure that your server remains in a working state. Sorting through the wide variety of security methods and options can take some time and consideration. It is, without doubt, best done with a preconceived plan that has taken into account all the different areas of vulnerability of the application to be secured.
One of the big advantages you will have derived from working on your store yourself is that you now have a fairly good idea of how the site works. Knowing how something works is a major step towards being able to secure it properly, because you intuitively have an idea for where vulnerabilities exist. This by itself is only a part of the security strategy, which should also encompass...