Generally, you cannot use Pell or other federal grants for graduate school. Federal financial aid via FAFSA generally includes loans, both subsidized and subsidized, as well as work study opportunities for low income students. You can learn more about federal financial aid offerings at https://studentaid.ed.gov/sa/sites/default/files/graduate-professional-funding-info.pdf
Depending on how competitive your law school or medical school application is, you may be able to get grants and scholarships directly from the institution from which you applied. In some cases, this may be enough to cover the total cost of education. You may also want to explore opportunities to be a teaching assistant ("TA") for an undergraduate course while you are in graduate school, as many universities offer a tuition discount and cash stipend in exchange for your work as a TA.
Please also note: in recent years some programs have announced they are offering all incoming students a full-tuition scholarship, like NYU's School of Medicine: https://med.nyu.edu/education/md-degree/md-admissions
Have you completed your undergraduate studies? If not, you will want to focus on getting the best grades possible while also being an active participant your community. High grades and test scores alone will not make your application stand out - you want to also show the graduate schools that you have interests outside of the classroom - and volunteering in the industry you'd like to work, or by just helping other people is a great way to do it! The next step is to ensure you meet the admissions requirements, including taking the entry exams (LSAT or MCAT). Your score on the entry exam will play a key role in your admissions, so I strongly suggest you take many practice exams and study to get the strong score possible.
I hope this was helpful. Please let me know if you need anything else.