Well it has a focal length of 1000mm so a 30mm will give you 33x magnification, a 20mm will give you 50x magnification, a 12.5 will give you 80x magnification, and a 6.5mm will give you 150x magnification, which I think would be well and truly the maximum it could cope with.
You divide the focal length by the eyepiece to get the magnification, so you can make you own choices there.
Personally I'd grab a 20mm and a 12.5mm and maybe the 6.5mm, but it depends what you already have.
Also, the scope probably needs collimating (lining up the mirrors) otherwise the view can be much lower quality than it should be:
http://www.celestron.com/c3/support3...eid=2310&nav=0
Once you get to know the scope, you may want to try some improvements to the collimation system detailed here:
http://www.astronomyforum.net/celest...er-127-eq.html
but you may want to get some local help with that, and I wouldn't clean the mirror unless it was absolutely filthy, easy to do more harm than good.