I think that every person has such situation, when you should meet with him or her parents. Yes, it is very difficult thing, especially if you do it at first time in your life. I think that many people have fears about this meeting. People have many bad thoughts, such as “They will not like me”, “I am so bad person”, “They will say that we do not fit each other” etc.
Yes, I know what it is in your mind at this time. You have mess in your head and you are very nervous. So let me give you pieces of advice.
Firstly, you should think about your lovely dearest person. You love your girlfriend/boyfriend. You have relationships with this person, so you should be confident that you would have nice meeting and relationship with him/her parents.
What you should wear for this meeting? Usually older people are old fashionable, so wear classic cloth. When you will see them, you will understand how to dress up in future. Maybe they will like your torn jeans.
I believe that everyone has different interests and hobbies. So tell them about your job, hobbies etc. Be yourself and do not lie. True is important thing in every relationships.
In the end, I want to say that, if you have bad contact with parents of your dearest person, so do not give up. You should be polite and do not concentrate your attention on bad things. Think about good! Stay positive!