Getting the red bracts requires a lot of effort and you won't get for this year now. Google 'growing a poinsettia' and you should get the info you need. Personally, given how cheap the plants are I don't even try to keep them going to produce new bracts.

If you want to try for sentimental reasons, keep the plant in TOTAL darkness--not even a flash of light--for around 14-16 hours each night. Depending on the variety, you will need to do this for 2-3 months, until the branch tips start to "bunch up" and the new leaves turn reddish. After that, it is usually 5-8 weeks until the plant is really showy. As MG says, usually easier to just buy a new one!