C function that counts lines in file

You declare

int countlines(char *filename)

to take a char * argument.

You call it like this

countlines(fp)

passing in a FILE *.

That is why you get that compile error.

You probably should change that second line to

countlines("Test.txt")

since you open the file in countlines

Your current code is attempting to open the file in two different places.

You have a ; at the end of the if. Change:

  if (fp == NULL);
  return 0;

to

  if (fp == NULL) 
    return 0;

while(!feof(fp))
{
  ch = fgetc(fp);
  if(ch == '\n')
  {
    lines++;
  }
}

But please note: Why is “while ( !feof (file) )” always wrong?.

You're opening a file, then passing the file pointer to a function that only wants a file name to open the file itself. You can simplify your call to;

void main(void)
{
  printf("LINES: %d\n",countlines("Test.txt"));
}

EDIT: You're changing the question around so it's very hard to answer; at first you got your change to main() wrong, you forgot that the first parameter is argc, so it crashed. Now you have the problem of;

if (fp == NULL);   // <-- note the extra semicolon that is the only thing 
                   //     that runs conditionally on the if 
  return 0;        // Always runs and returns 0

which will always return 0. Remove that extra semicolon, and you should get a reasonable count.