First you are trying to write to the innerHTML of the input field. This will not work. You need to have a div or span to write to. Try something like:
First_Name
<input type=text id=fname name=fname onblur="validate()"> </input>
<div id="fname_error"></div>
Then change your validate function to read
if(myform.fname.value.length==0)
{
document.getElementById("fname_error").innerHTML="this is invalid name ";
}
Second, I'm always hesitant about using onBlur for this kind of thing. It is possible to submit a form without exiting the field (e.g. return key) in which case your validation code will not be executed. I prefer to run the validation from the button that submits the form and then call the submit() from within the function only if the document has passed validation.