Here is the entire program(with comments):
Code:
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

void rectangle(); //function that will draw a rectangle with dimensions provided by the user
void number(); //function for printing the sum of positive integers whose digits are rising from left to right

void main()
{
    int menu = 1; //while this variable is set to 1, the loop that shows the menu continues
    int choice = 0; //a variable to store the menu choice the user makes
    char exit = 'n'; //stores the choice the user makes in the 3rd menu item

    while(menu == 1)
    {
        while(choice <= 0 || choice > 3) //if the user enters an invalid value, the menu will appear again
        {
        printf("\nChoose an item from the menu:\n");
        printf("1. Draw a rectangle!\n");
        printf("2. Calculate a sum!\n");
        printf("3. Exit?\n");
        
        fflush(stdin);
        scanf("%d", &choice);
        }
    if(choice == 1) //for each choice, except 3, the appropriate function is executed
        rectangle();
    if(choice == 2)
        number();
    if(choice == 3) //this choice is built into the main function
    {
        printf("Do you really want to quit? Press 'y' or 'Y' to confirm, any other key to cancel");
        scanf("%c", exit);
        if(exit == 'Y' || exit == 'y')
            menu = 0;
    }
    }        
}

void rectangle()
{
    int length, width; //dimensions of the rectangle
    int dimensions;  //variable to check if the coordinates of the symbol are okay
    int row, col; //row and column coordinates of the symbol
    int symbol_count; //how many - were printed
    int row_count; //used to track how many rows were printed in the rectangle
    char symbol = 'n'; //the symbol the user chooses to print inside the coordinates
    
    printf("Please enter the length and width of the rectangle, seperated by a space\n"); //requesting the dimensions of the rectangle from the user
    scanf("%d %d",&length, &width);

    if(length < 0) //checking if the coordinates of the rectangle given by the user are negative
        length = -length;
    if(width < 0)
        width = -width;

    dimensions = 0;
    while(dimensions == 0) //checking if the coordinates given for the symbol are not negative or too big
        {
            printf("Please enter the row and column coordinates of a point in the rectangle,\n seperated by a space\n");
            scanf("%d %d",&row, &col);
            
            if(row >= 0 && row <= length)
            {
                if(col >=0 && col <= width)
                    dimensions = 1;
                else
                    printf("Invalid column coordinates\n");
            }
            else
                printf("Invalid row coordinates\n");
        }

    printf("Please enter a character to be printed in the rectangle\n"); //taking the character to be printed in the rectangle
    fflush(stdin);
    scanf("%c", symbol);

    symbol_count = 0;
    row_count = 0;

    while(symbol_count < width*length) //once the symbol_count reaches the dimensions of the rectangle, the loop will break
    {
        if(symbol_count == col)
            if(row_count == row)
                printf("%c", symbol);
        else if(symbol_count == width)
            printf("\n");
        else 
            printf("-");
        symbol_count++;
    }
}

void number()
{
    int num; //this is the original number, which will be printed if it meets the criterion
    int num_test; //the number will be working on in the function
    int num_ok = 2; //the flag used to mark the number as good for adding to the sum that will be printed
    int mod1, mod2; //variables that will be used to compare the digits of the number
    int sum = 0; //the sum of "good" numbers, which is set to zero here and will be added to in the function.
    
    while(num > 0) //the loop will continue for as long as the user enters positive numbers
    {
    printf("Please insert an integer:\n"); //taking the original integer
    fflush(stdin);
    scanf("%d",&num);
    
    num_test = num; //setting test_number to the value of num, as entered by the user

    while(num_test > 0 && num_ok != 0) //this loop will quit if the user enters a non-positive zero or a number with digits that aren't rising
    {
        mod1 = num_test%10;
        mod2 = (num_test/10)%10;
        if(mod2 > mod1) //once the user enters a number with digits that aren't rising, the expression in the if statement will be false and num_ok will be set to zero, which will break the loop
            num_ok = 1; //if each of the digits passes the test, num_ok will remain equal to 1
        else num_ok = 0;
        

        if(num_test == 0 && num_ok == 1); //if the division reached the final stage(num_test is equal to zero, and the num_ok flag is still equal to 1, then the number is valid and will be added to the sum 
            sum = sum + num;
    }
    
    printf("%d\n", sum); //printing the final sum
}
}
It compiles but crashes on the first menu item when entering the character to be printed... (maybe it crashes later too, haven't checked yet)

Advice, anyone?