function validate_form() {
    valid = true;

    if (document.getElementById("first_name").value=="First Name*") {
        alert ("You must enter your first name." );
        valid = false;
	} else {
		 	if (document.getElementById("last_name").value=="Last Name*") {
				alert ( "You must enter your last name." );
				valid = false;
			} else {
					if (document.getElementById("email").value=="Email*") {
						alert ( "You must enter your email." );
						valid = false;
					} else {
							if (document.getElementById("contact_number").value=="Contact Number*") {
								alert ( "You must enter your contact number." );
								valid = false;
							}
					}
			}
	}

    if (document.getElementById("first_name").value=="Ime*") {
        alert ("Unesite ime.");
        valid = false;
	} else {
		 	if (document.getElementById("last_name").value=="Prezime*") {
				alert ("Unesite prezime.");
				valid = false;
			} else {
					if (document.getElementById("email").value=="E-mail*") {
						alert ("Unesite e-mail.");
						valid = false;
					} else {
							if (document.getElementById("contact_number").value=="Kontakt telefon*") {
								alert ("Unesite kontakt telefon.");
								valid = false;
							}
					}
			}
	}

	return valid;
}