I am an 8th Grade student and I have a lot of interest in Biology subject. But am very bad at drawing. My friends said that if i want to pursue a career in medicine i will face problem if I cant draw diagrams of suppose internal structure of heart etc

That's excellent that you are planning ahead for your future! And it's great that you love biology :) There's a lot that you can do in the field of biology. Although there is some drawing (mostly in anatomy courses and a bit in biology/physiology), you don't need to be an artist. If you can draw a stick figure, you should be fine. If you take AP chemistry in high school, they'll usually let you use a ruler anyway to draw molecular bonds. Think of high school as an audition for college; and college as an audition for a doctorate degree. The greatest of actors today all have stories about the times they've been rejected for a part. So don't let one little thing like drawing discourage you. I honestly don't know any art majors that turned pre-med. While in high school, focus on keeping your grades up (especially in the math and sciences) so you have options for college. Maybe in sophomore or junior year, start getting involved in extra curriculars like volunteering at a hospital or shadowing a doctor, so you know firsthand if being a doctor is really what you want to do.

It is much more important to recognize the things you see than being able to draw them. These are two very different skills.

For example, you mentioned drawing the heart. Most cardiologists and cardiac surgeons I know carry around a pad with printed drawings / diagrams of the heart. They will then make annotations on these printed diagrams to explain to a patient where a blockage is or where an incision is made.

As far as making sketches or drawings, it is much more common to take photographs to preserve an image,

I second Dianna, although having a good drawing skill can be handy when trying to portrait a organ in Anatomy/Physiology classes, its not necessary to super artistic about it. Goal is to visualize the organ or organ systems and try to piece the blocks in your mind on how the human body is finely tuned and every single component works in harmony with each other. As far as pharmacy goes, the ability to visualize these organ system helps us pharmacist learn in dept about the role of drugs/medication play to modulate these organs to get a desirable result or side effects. So, in conclusion drawing skill is essential tool to learn biology not a criteria for it.