If you want to go there by flight, the closest Airport is in Dehradoon. There are multiple direct flights from Delhi, Lucknow and Mumbai.
By CarCar is the most popular option people prefer. From Delhi, it takes around 7 hours to reach Rishikesh.
By TrainThere are not much trains to Rishikesh, but you can reach Haridwar easily. From haridwar, you can take any local train, bus or cab.
By BusThere are several government and private buses operating between Delhi to Rishikesh. AC buses are also available.