There are various options which you can choose according to your convenience.
By air- There are no direct flights available from Delhi to Manali. Take a flight from Delhi to Bhuntar. After landing to Bhuntar, you can take taxi or bus to go to Manali.
By train- Travelling by train can be quite hectic as you need to travel to Chandigarh and then take another train from Chandigarh to Manali.
By Bus- You can take a Volvo bus from Delhi to Manali.
By car- If you are good at driving then take a car as this is the most preferred option to go to Manali from Delhi. You can enjoy beautiful surroundings and can stop at any of these to capture beautiful sights. If you are not good at driving to hilly regions then I would suggest you hire a cab.