Well, to start, setup a space for your string for 11 bytes(trailing zero) and put that in your data section. Also, put your string prompts into the .data section. (i.e. "Enter 10 character string: ") Then, in your main program, you print the prompting string (you know how to use syscall, right?) Point the $A0 at the input string buffer. Use the syscall for reading an input string.
Repeat this block for characters 2, 4, 6 Print out the first output text ("The 3rd character: ") Load an immediate 2 (being the 3rd number) into a temporary register like $t0. Use a load byte for $a0 from your input string offset by that register. Use the print byte syscall. Repeat (or just copy/paste)
And syscall to exit. Happy coding!
EDIT: For sanity's sake, start your "The 3rd character: " with New Lines (i.e. "backslash n"). That way, when you print you character, and then the next string, it won't be jammed together.