How do I insulate my garage ceiling?
I just did closed-cell spray foam in my attached garage. 



There is a bedroom above (to the left of the I-beam in the photo), while the front sloping part is a hot roof (see Do I need to add roof vents if I close off a small attic space in the garage? for detail/pics). 



In your case, you should decide if you want the attic space to be 'conditioned' space or not -- You should not have insulation in both the ceiling and on the roof. That will just create different thermal zones, which can mean you have different places for condensation to form, which means you can get mold. Without actually conditioning (heating or AC) the garage itself, which one is hotter and/or more humid at any time will change throughout the year, and I think having two layers of insulation is just going to cause problems.

I'm not sure what you mean by 'aluminum sheets' -- perhaps you are referring to foil-faced Polyisocyanurate? The foil in that case forms a radiant barrier -- in the case of attic space, I'm not clear on if this would actually make a noticeable benefit. Polyiso does have a very high R-value per inch (near that of closed-cell foam) so it may make a good choice anyway regardless of the foil -- but cost and value are at issue (I'm not sure how it compares cost-wise to regular XPS). 

Keep in mind that if you're using any type of foam boards between rafters, that the rafter itself will not be insulated, so you only get the insulation value of the wood itself. This is one of the benefits of spray foam (and the reason I went with it) -- you get a complete thermal break, and it fills all the crevices. Expensive, but they did my entire garage in about 4 hours (including setting up and cleaning up, and encasing some duct work that hangs below the ceiling joists).