The DV program creates 50,000 visas every single year and they are drawn at random for individuals who are members of the countries that have a low immigration rate. A lot of the lottery winners are outside the US and they come in with their own immigration visa. Some of the people who win the lottery are residing in the states as a non-immigrant and when this happens, USCIS processes the status applications. If you want to prove that you are eligible then you need to make sure that you have a visa available at the time you file the application form. This is an I-485 and you also need to prove that you are admissible to the United States as well.
It is actually possible to see how many visas are available by checking the DOS via bulletin board. If you go down to section B then you will find that there is an immigrant category and the chart on there shows you where the cut off point is and whether or not it has been met. When the cut off point has been met then the visas that are available will be ranked according to numbers for the geographical areas.
If you want to apply for a green card then you need to make sure that you have submitted the right form with proof that you are between 14 and 79 years old. You also need two passport photos, an original copy of your birth certificate and a record of your vaccinations as well. There may be other forms of documentation that you need however these will depend on your personal circumstances and what information you have already provided the office with. If you want to find out more about this, contact your visa advisor today.