The Imperial Palace Grounds

To the palace itself you will not be able to see unless you apply to a free guided tour that the Imperial Palace Agency offers the public.
Imperial Palace Agency Site: Imperial Palace Agency

These tours are conducted from Monday to Friday, except for national holidays, 28 December to 4 January.
The tour takes 75 minutes and they are available from 10:00 to 13:30. The tours are in Japanese, and for foreign visitors, there is a brochure (English, Chinese and Korean) or Audio Guide in English.

Reservations for tour of the Imperial Palace must be made few days before the planned visit.
Take into consideration that every tourist spots are closing very early in Japan, and the tour application are accepted Monday to Friday, 08:45 to 16:30. You can make them by phone can be made by telephone: 03-3213-1111 (Ext. 485, 486) Imperial Palace Tour office, or by visiting this site http://sankan.kunaicho.go.jp/order/index_EN.html
After applying you must pick up the permission card at least 1 day in advance to the date of your visit.
Finally on you visit day you should enter the Palace through the Sakashitamon Gate with passport or another form of identification.