Everyone enjoys being complimented, and your boyfriend's parents are no exception. If you like the style of their house or the paintings on the wall, feel free to say so. It's also a good way to start a conversation, because you can ask where they found a particular item. If your boyfriend's mom cooked, compliment the meal. If his dad chose the restaurant where you're eating, compliment that. It doesn't have to be any big deal, just a casual "I'm glad you picked this place; it's great!" will do.
Your compliments need to be sincere, however. If you lie about loving Mom's earrings or collection of penguin knickknacks, you might find yourself the recipient of something similar -- forcing you to continue the lie. Also avoid gushing about everything, which you might be tempted to do out of nervousness. You'll come off as fake and your boyfriend's parents will question your sincerity about everything.
Meeting your boyfriend's parents for the first time doesn't have to be scary. Just be the nicest, politest, friendliest version of yourself and you'll be fine. You can't make them like you, but you can leave them with a good impression.