Currently a sophomore at a top 10 Engineering school.
I slacked my freshman year, but im still above a 3.0, so my question is how important would freshman and sophomore grades be to graduate schools.
Also, what can i start doing to maximise my chances at getting into a top 10 mechanical engineering grad school?
Important, but less important than junior and senior grades. Different schools handle admissions in different ways, some look at your entire GPA, some look only at your last 60 credits, so do the math and see how important your grades are in those two circumstances.
Top 10 schools have their pick of students. If you want to get in, you need a rock star GPA and a solid research record. So bring the GPA up and try to get involved as soon as possible with academic research and stick with it through the end of school, trying to get to the point where at least 3 professors think you are a great potential grad student. Publishing helps a lot, but is not vital.