The written score needed to pass CDL test requirements is an 80. Common mistakes that people make on the written portion usually involve overthinking and second-guessing. The main thing you’ll want to do is trust your instincts and be confident in your knowledge.
As for the driving test, you’ll want to avoid automatic fails, like hitting curbs, forgetting to use your turn signal when changing lanes, not checking your mirrors, rolling backwards from a stop, and running a red light. Other common mistakes that aren’t automatic fails include missing a shift and taking a turn too wide due to being overly cautious. These won’t count against you as much as automatic fails, but they are things your instructor will keep an eye out for.