I've been in America for 18 years, how can I go about becoming legal in the United States?

Not enough information.

Where are you from originally? How did you enter the US? What have you filed with USCIS? Do you have any USC relatives? Do you have an approved labor certification? What is your educational background and employment history? Any arrests, charges or convictions? Any immigration violations?

Are you married? Do you have children? What is their status?

This is the basic information needed to start assessing your immigration eligibility.