When to take a test?

So me and my boyfriend have intercourse every so often and the last time we had sex was the 18th of November and I'm not sure how pregnancy tests work but lately I've been nauseous, having mood swings, and cravings but I wasn't sure if they were possible pregnancy symptoms or just out of the blue type of stuff. I want to take a test but I don't know if it's too early to determine whether I would or wouldn't be pregnant. I'm very observant and cautious but I don't know if I'm over thinking it all. Hope someone can help me! Lol