How to make a find function in godot

For example: If blablablub IN string: do something

so how to say godot, if he finds a word combo in a string then do something?

That's pretty much how it works :)

if "blablablub" in "string":
    print('This will never evaluate to `true`, since the string "blablablub" is not contained in "string".')

The check is case-sensitive. To perform the check in a case-insensitive manner, use if "blablablub".to_lower() in "string".to_lower().