It's okay, take a deep breath and relax. Simply complete a purchase as a guest and you will be provided with details to create an account once you have completed your purchase.
If you have already made a purchase but did not make an account you will be offered to create an account on your next purchase.
Previous orders will show up in your account as long as you use the same email address.
There are many perks to having an account.
1) Your shipping and billing details will be saved so you don't have to type them in on your next purchase.
2) You can see a history of your orders which makes it easy to retrieve order numbers for tracking shipments etc.
3) You'll get a warm fuzzy feeling inside ;)