Start at Mossdeep City. Go to the southern part of city and go down to Route 127. Go down that route until you make it to Route 128. Once you've made it to Route 128, head east until you reach Ever Grande City. Head up the waterfall and get ready at the Pokemon Centre; Victory Road is right in front of you.
Don't forget to face all the trainers on the routes as they will provide valuable experience.
Hope I helped. :)
Source: Following the routes' maps