Summarize the following:
This is your first girlfriend, and you want your parents to understand how much she means to you. However, this doesn't mean acting the same way you do around your girlfriend when you're both alone together. Overdone PDA can cause the interaction to be a lot more uncomfortable for your parents, and it could affect their view of the relationship.  If you want to show your girlfriend your support and affection, put your arm around her or give her hand a squeeze. This is a mature level of physical interaction between a couple and will not make your parents uncomfortable. Avoid prolonged kissing or touching. Although this might be normal for the two of you around others, this is not a good way to act around your parents. They will appreciate your maturity if you abstain from this kind of behavior around them. It's important to show your parents that you respect your girlfriend. Do not tease, yell or swear around her in any way. You may be feeling vulnerable or even confused about how to act in this interaction. It's important to remember that your girlfriend is probably feeling the same way! Make sure that you treat her with support and respect and make her proud to be dating you. Although you and your girlfriend may talk together about every topic under the sun, there are just certain things that your parents don't want to hear. Keep everything G-rated when they are around. Talk to your girlfriend in the same way your would talk to a close friend and keep the conversation family friendly.
Don't overdo the PDA. Act respectfully towards your girlfriend. Don't talk about personal things with your parents around.