Creating a Star Citizen account on the Roberts Space Industries (RSI) site is without obligation to purchase and will allow you to be eligible for free flight events that are held regularly throughout the year, to allow you to test the game.
When you create your account, a referral code will be required.
This one is not mandatory, but specifying a referral code will give you a bonus of 5000 UEC (United Earth Credits), the currency in play of Star Citizen.
If you don't have a referral code, you can use this one: STAR-QP2M-YLS3, and paste and copy it into your enlist formulary.
Or follow this link to create your account: Enlist Formulary with STAR-QP2M-YLS3 allready in place.
This referral code can only be used at the time of Star Citizen account creation and can not be added retroactively to a pre-existing account, so you have every interest in doing so now.