Wilton makes a "baby buggy" pan. But, if you'd like to shape it yourself, Use a large round cake. Slice the cake in half to form the bottom. Cut 1/3 of the remaining half for the awning. Depending on the size of the buggy, the wheels can be small round cakes or purchased round snack cakes such as ring dings
Try doing a search on google,ask,food network, or even youtube. Youtube I believe is the best one to visit because they may have a video with someone giving step by step instructions on how to make a baby buggy cake
The simplest way would to be to purchase a cake mold in that shape. Or you can take a sheet cake and cut it out yourself and then decorate to your liking. There are also many suggestions and directions in magazines, cookbooks or online. It requires attention to detail and patience, good luck!