If I get a mortgage loan, will that make my credit go down?

As long as you make your payments on time it should actually help improve your credit score. If you have late payments then it can negatively affect your credit score.

Generally speaking, anytime you take on new large debt, it has the potential to temporarily lower your score slightly while it figures out how you handle the new debt. But realistically, no - it should only help as long as you make on time payments. But the bigger item to talk about is the complete paranoia the general public has about credit scores. 99% of the time for 99% of people, just pay your debts on time, and you'll never ever have to worry about credit scores.