Generally you're going to say "call" all the time, but "call up" is used occasionally in casual settings, like "I'm gonna call up my friend, Mark." You don't have to use it, but just know it means the same thing.

Lauren is right on the formal/informal distinction, but also should mention that “call up” means on the phone and “call” is more general. It can be on the phone or otherwise “I’m going to go outside and call the kids.” (using a loud voice.) You wouldn’t say “call up” here.