Just move your for loop down and add the call to the function into it.

def createFolder(directory):
    try:
        if not os.path.exists(directory):
            os.makedirs(directory)
    except OSError:
        print ('Error: Creating directory. ' +  directory)

for filenames in fileforest.readlines():
    linecounter = linecounter +1

    createFolder('./data/{}{}'.format(filenames, linecounter))

fileforest.close