my brand new cello plays every string too low! I've tightened my A string until it was very firm and it plays lower than my G string! I don't think that's good because my D and G string are playing as low as a bass. does anyone know why?

Well, some not-so brilliant person could have used strings for a bass (maybe a quarter sized one) Or it could be strung wrong... If you can, compare the string thickness with another cello's. But new strings might be a good idea. Where did you get the cello? You might ask the store/previous owner.