Draw a long narrow triangle on its side. (Use long narrow triangles unless you want a fat tuna).
Draw a second triangle also on its side, only slightly longer, with the bases touching.
Draw a long skinny triangle touching tips with the triangle on the right.
Draw another triangle that directly reflects the last one drawn to make the tail.
On the top of the second triangle just behind the vertical line draw a moderate size triangle to form the dorsal fin.
Just behind the main dorsal fin draw a smaller version forming the secondary dorsal fin. Below on the underbelly draw a fin reflecting the second dorsal fin to form the pelvic fin.
On the underside of the first triangle(head) draw a moderate size fin pointing downward to form the shark’s caudal fin.
Just a bit forward of the caudal fin draw a triangle pointing forward to form the bottom jaw.
Draw little triangles in the mouth of the shark to make the teeth.
Draw little circle forming the eye at the front of the shark.
Finally draw five lines to form the gills just above the cadual fin.
Feel free to add more detail when comfortable with your drawing.
Add shading for those who advanced enough to move beyond a simple doodle.