Does the front tyre "vibrate" if it is not on straight as others have said it will be out of balance.
But pulling one way sounds like wheel/fork alignment.
Can the front wheel spacers be fitted on the wrong side? are the brakes dragging/ binding.
Is the front wheel pointing fwds with the bars straight ,sit on it and look,think centerlines.
As Kwacker boy said is back in straight, look at chain adjuster marks for a quick and dirty check.
What back tyre do you have on?