Unlike other countries there is no such thing as a green card for UK. There are a number of immigrant visas available. You can apply for a green card through online from http://www.visa4uk.fco.gov.uk/.
Guidance on every type of UK visa is available here http://www.ukvisas.gov.uk/en/howtoapply/… and all the information on points-based visas is down the bottom of the page.
Following are the requirements to get a UK citizenship:
The person should stay for at least 5 years before the date of your application (unless your spouse or civil partner is a British citizen)
Should not spent more than 450 days outside the UK during those 5 years
Should not spent more than 90 days outside the UK in the last 12 months
Been granted indefinite leave to stay in the UK
Had indefinite leave to stay in the UK for the last 12 months