If you have chosen Squirtle as your starter then you will be able to catch Raikou. To do so stock up on Max Repels and get a Lv49 or under pokemon. Use the repels near Route 1 or 2 and eventually Raikou will appear. This works on any route however the repels deter any low levelled pokemon from appearing. Raikou roams across Kanto. If you chose Charmander however then you will be catching a Suicune and if you chose Bulbasaur you will be seeing Entei.