People tell me that a score of 188 on the PSAT for a 10th grader is good. What about a score of 30 on the PLAN?

those are both good. I got 178 on the PSAT and 31 on the PLAN when I was in 10th grade, and ended up getting a 218 on the PSAT junior year, 2210 on the SAT, and 35 on the ACT. Just make sure you continue to prepare and you should do great!