There are two parts in the process of getting a green card. 

The first part is to file an immigration petition which establishes an eligibility for you to obtain the green card. 

The second part is adjustment of status which issues you the green card.

For employment-based applications and some family-based applications, these two parts can be filed at the same time.

When you file an immigration petition, the USCIS will give you a priority date which determines when you can file for adjustment of status. Before you file for adjustment of status, you have to maintain a legal status (F, J, H or other valid visas). The immigration law specifies a maximum of 20,000 immigrants per year per country for all categories of immigration. (Categories are ranks of preference for different groups of people). When the total number of people applying for green card exceeds 20,000 for a country, a waiting line appears. Even you are going to hire an attorney to do it for you, it would be beneficial to you if you know the procedures and apply earlier. An attorney can file petition (filling out the forms) for you. But you have to establish the eligibility yourself. For details, please click the link above.