How can I improve my grammar.?

When I was placed into sp. ed in first grade and not placed into regular ed until 8th grade. I never learned the whole grammar thing. I am 19 years old believe it or not I am actually very smart. I graduated high school with a 3.0 gpa, I love to write but I obviously suck at it. Please I just really need a site that can enhance my writing skills before I go to college.