Simple Problem about forms and sending info to db

I am currently coding a cms. So far I have completed the security login/password system but have now moved on to a new area of php. Uploading info to a database. This is the code as it stands now. <?php if($_GET[action] == ""){echo" <form name="adduser" method="post" action="adduser.php?action=adduser"> <table width="400" border="0" cellspacing="0" cellpadding="5"> <tr valign="top"> <td width="50%" align="right">Full Name:</td> <td width="50%"><input name="fullname" type="text" id="fullname"></td> </tr> <tr valign="top"> <td height="50" align="right">Email:</td> <td><input name="email" type="text" id="email"></td> </tr> <tr valign="top"> <td align="right">UserName:</td> <td><input name="username" type="text" id="username"></td> </tr> <tr valign="top"> <td align="right">Password:</td> <td><input name="password" type="password" id="password"></td> </tr> <tr align="center"> <td colspan="2"><input type="submit" name="Submit" value="Submit"></td> </tr> </table> </form> ";} if($_GET[action] == "adduser"){echo "Add User Function is still Under Construction <br> as a result what you just did has had no effect"} ?> I know there is something wrong as i get an error when I goto the page. I want it to display the form, then when you click submit, goto the same page except run the command to add the information from the form to the database. Also how do I put the information in the database. Regards Logan