Besides the bug that Richard has already pointed out and the bracket problems that Dave has already partially fixed, there are a few more things that don't work well. I have written the error locations as comments in the source code.
int main(void)
{
int c; float n = 0, baja, alta;
for (c = 1; c <= 20; c++)
{
printf("%i-. Ingresa calificacion:\n\n", c);
scanf("%f", &n);
if (c == 1) {
alta = n;
baja = n;
}
else {
if (n<baja) {
baja = n;
}
if (n>alta) {
alta = n;
}
}
}
And then one more note:
_getch();
My suggestion would be to dispense with both conio.h and getch() and instead use getchar() to wait for user input.