Creatine is a terrible idea, do not touch the stuff for cycling.
Intervals are the way, there is far more science to it than just sprinting up a hill for 20 seconds. Either buy a HRM or a powermeter. Work in zones to improve various aspects of your fitness, muscles tend to develop in harmony with the rest of your system for a particular task.
Some intervals functional threshold HR/Power, 20mins On – 10mins Off (slow) – 20mins On
Some high intensity – 5mins On – 2mins Off etc…
Some killer – 20seconds On – 3mis off etc…
As for supplements; A good energy drink like high5 without Aspartame, and a good quality protein powder (100g per day spaced out) like Reflex instant Whey.
Best advice; Ride 3 times a week with a rest in between, that’s two hard rides and one easy ride. Easy ride = 30-40 MTB miles, mega slow, take some lunch and a camera, walkers should be passing you. Hard Ride – Near death experience, start slow and build up, should last no more than 90 mins. Finally; if it isn’t nailed down eat it.