@DLedoux, in the example you gave of a lie, it is the content which determines the gravity (amongst other things). So if you were under legal age, living at home, and your mother asked you if you’d finished your homework and you told her yes (when knowing full well you hadn’t), then that would be a venial matter (finising the homework), and if your mother asked you if you went to school today and you said yes, when in fact you didn’t but went to the park with friends etc., then that would be graver than the unfinished homework.
As I understand it , whilst both are lies told to your mother, one is more serious than the other - unfinished homework versus wagging school for the day.
But I suggest you make an effort to form a friendship with your parish priest, then you will comfortable in asking him your questions, and he will be better placed to help you with any issues you have in coming to understand how to work out the gravity of any particular situation/sin.