I was flying Southwest airlines going from KBUR (Burbank) to KSMF (Sacramento). After take-off I was told to maintain 1000 ft. I think I went up to 10000 ft because I thought that was the correct altitude. But they keep on saying to go to 1000 ft so on the mini map I dragged the altitude slider to 1000 I crashed into the side of a mountain! Do I have to set an altitude when I am filling out my flight plan? I love using ATC and I want to keep using it so pleas help!!