I currently use the .startsWith to find out if the tweet starts with the same words as makeCoffee in order to recognise a tweeted command.
Use the strncmp() function, instead.
how do I complete the function with currentLine as a char?
You can't. currentLine needs to be a char array. And, you need an index into the array, to keep track of where to write.
char currentLine[80]; // Reserve space for some characters
byte index = 0;
currentLine[index++] = inChar; // Append the character
currentLine[index] = '\0'; // NULL terminate the array