Welcome to CS, Chippie13310! ^^
Things can be confusing at first, but don't worry, you'll have it all figured out in no time. C:
To lock a group, go to the group you want to lock. At the bottom of the page, there is a link that says "Change trading settings for this group." Click that, click "Nobody can make trade offers for these pets" and then hit "Save Settings." ^^
Whenever you have a question, the best people to ask are the
Newbie Helpers (they're easy to identify because all of them have salmon-colored names). They're all really friendly and will do their best to answer all of your questions. C: