Pick a good day when you know your parents won't be busy or distracted from the day's events. In fact, pick a day when you know they'll be the most relaxed. Try to do this within your house and not in a public place since you do not want any embarassing public scenes to occur. Then tell your parents to sit down and listen to something important that you want to tell them. In these situations, it's always better to be prepared for the worst. Anticipate that your parents might be very upset and plan accordingly.
The fact that you're hiding the truth from your parents means you probably think that they oppose the idea of lesbianism. If you want to have a feel of how they would react, you could try discussing the topic with them - casually mention that you have someone in work or at school who is in a lesbian relationship but are being pressured to break it off. Then ask your parents what they think of it. Their reply should tell you whether you should continue to consider telling them.
You know your parents best, so you can probably guess how they will react. However, you may be surprised what they already know about you (or at least suspect), and you may not really be hiding anything at all. Consider the alternative to telling your folks. How long can you continue to keep them in the dark? If it is inevitable that you tell them the truth, find a way to do it where you will have the support you need, no matter how they react. Plan what you’ll do before the meeting to keep yourself grounded and afterward to get the emotional encouragement you may need.