First you need to have the Xbox Live gold membership (silver membership is the default). The gold membership costs about $60 per year. Next you need a netflix membership, which is about $10 a month depending on which plan you select. If you have both then you can go to netflix's website to find the exact process for enabling it on your xbox.
If you don't have Xbox live gold, then you can also stream netflix to a Wii without needing an additional online membership.