Shibuya is on the JR Yamanote line between Harajuku and Meguro stations. It can easily be reached on foot from Harajuku and Omotesando. From Omotesanda to Shibuya it is a pleasant walk along Aoyama-dori. Roppongi is a slightly longer walk, but there are several buses which run along Roppongi-dori.
Map reproduced with permission from Tokyo Convention & Visitor Bureau www.tcvb.or.jp