Start your script with She-Bang header:
#!/bin/bash

Variables are assigned by a key-value pairing:
FILENAME="~/docs/input"

You can print to the console by using echo:
echo $FILENAME

An IF statement:
if [ ! -s $FILENAME ] ; then
  touch $FILENAME
fi
The bang ! negates the result, and the -s parameter checks if a file exists.

Example of a function:
function getFile
{
  # function contents here
  # the hash symbol makes the rest of this line a comment
}

Call this function by using its name:
getFile

You can pass a variable to a function:
getFile "/home/user/docs/file.txt"

And use it within the function by reference to $1
function getFile
{
  echo $1
}