help with inserting data into mysql

could someone help me i can't seem to insert data into mysql table and don't know whats gone wrong

//create_cat.php  

include 'mysql.php';  
include 'header.php';  
echo '<h2>Create a topic</h2>';  

if($_SESSION['loggedIn'] == false)  
{  
    //the user is not signed in  
    echo 'Sorry, you have to be <a href="signin.php">signed in</a> to create a topic.';  
}  
else  
{  
    //the user is signed in  
    if($_SERVER['REQUEST_METHOD'] != 'POST')  
    {  

        //dropdown is being used here where we'll retrieve the catagories from the database for use in the dropdown  
        $sql = "SELECT 
                    id, 
                    name, 
                    description 
                FROM 
                    catagories";  
        $result = mysql_query($sql);  
        if(!$result)  
        {  
           //query did not work
            echo 'Error while selecting from database. Please try again later.'; 
        } 
        else 
        { 
            if(mysql_num_rows($result) == 0) 
            { 
                //there are no catagories, so a topic can't be posted  
                if($_SESSION['user_level'] == 1)  
                {  
                    echo 'You have not created catagories yet.';  
                }  
                else  
                {  
                    echo 'Before you can post a topic, you must wait for an admin to create some catagories.';  
                }  
            }  
            else  
            {  
                echo '<form method="post" action=""> 
                    Subject: <input type="text" name="subject" /> 
                    Category:';  
                echo '<select name="cat">';  
                    while($row = mysql_fetch_assoc($result))  
                    {  
                        echo '<option value="' . $row['id'] . '">' . $row['name'] . '</option>';  
                    }  
                echo '</select>';  
                echo 'Message: <textarea name="post_content" /></textarea> 
                    <input type="submit" value="Create topic" /> 
                 </form>';  
            }  
        }  
    }