Uninitialized local variables

As a C newbie, your #1 error will be forgetting to initialize
variables.

squaresum.c

#include <stdio.h>

int squaresum(void) {
    int sum; /* Never set to 0 */
    int i;

    for ( i = 1; i <= 10; ++i ) {
        sum += i*i;
    }

    return sum;
}

int main( void ) {
    printf( "1st time, %d\n", squaresum() );
    printf( "2nd time, %d\n", squaresum() );
    return 0;
}

$ squaresum

1st time, -1073744027
2nd time, 385

Your results will vary, because things are differently random on your machine.

Jack up your warnings. Always compile with -Wall. Some warnings
may only get returned with the -O flag on.