This website is quite good with solution descriptions but lacks diagrams - sorry can't find one with diagrams.
http://www.diynot.com/pages/pl/pl030.php
If the top of the tap is just wobbly on its spindle, then try using Threadlock or similar product from Halfords to 'lock' the little screw that (usually) holds it.
If the tap top plus spindle is too stiff on the upper body of the tap that includes the O-ring which seals the spindle / body, it is presumably turning the upper body in relationship to the lower body? If so, you will have to take the whole upper body off (large spanner required on hex part) and find out why it is so tight. Probably gummed up with calcium. Take it all apart, and reassemble using the link above description about changing the O-ring.
You tap may be a different sort from my assumption - so sorry if this is not the way your tap works (taps with ceramic discs inside are different, for example).