Filters in C

Hi everyone. I have some problems. I just start my work with C programming that’s why I have problem with creating even simply program. In next week I will have a exam and the teacher gives as to do a list of program which probably are similar to those on exam. Maybe someone can know how to do one of this program below?? This is the filters which I can later use it in CMD to filtering some .txt file.

List:
- Change all lowercase characters to uppercase characters and vice versa.
- Calculate the number of all words starting with an uppercase character.
- Change all characters to uppercase characters.
- Select all lines shorter then n characters.
- Select all lines with the number of characters from n to m.
- Select all lines starting with a given character.
- Select all lines starting with a given string.
- Write a filter that displays the number of occurrences of characters 'a' 'b' 'c' 'd' and 'e' in the standard input.
- Write a filter that displays the number of occurrences of all characters from the
standard input.

Please help me.

Start simple, say with the filter which does nothing except copy the input to the output.

All the other ones would be based on that, and it's an easy one to get right from the beginning.