To store text in a file in the same directory as your robot script:

*** Settings ***
Library    OperatingSystem


*** Tasks ***
My Task
    ${my_file_path}=    Set Variable    my_file.txt
    Create File    ${my_file_path}    Hello World!
    # creates my_file.txt with content Hello World!

Robot framework uses UTF-8 as default file encoding. If you need a different encoding you can specify it like this:

*** Settings ***
Library    OperatingSystem


*** Tasks ***
My Task 
    Create File
    ...    path=my_file.txt
    ...    content=Hello World!
    ...    encoding=cp1250