validating that enter character should not be text using javascript

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ASP.NET

hi i have a textbox i want that user should not be able to enter character
only numeric value allowed using java script .i try the code as

function checkforinteger(e)
{var a=document.getElementById("textbox1");
if(window.event)
{a=e.keycode
}

}

also i want that user will not b able go to next textbox till he correct
the error.i think it is possible using onblur.but i dont know how to write it using javascript code .i m very very new to javascript .plz help. also give me some tips how can i improved my javascript with site should i follow