how to make a letter to number converter in java?

i have a coding assignment to make letters turn to the corresponding number in java. i don't know where to start i will attach the instructions and the given code to see if anyone can give me some direction. my issue is that i can get it to do it for the lowercase numbers but when u try a uppercase it comes out a totally different number. i want 'A' and 'a' to both correspond to 1 but i'm not sure how to do that. i am also having trouble writing the if statement that will print "invalid input if the input in not a letter