Because tuna is generally sold in big chunks, you can ask for the piece that is the fattiest part of the belly (it is a pale light pink).
Cut the fattiest part into slices, 1 inch thick. Then, cut diagonally across the lines that are on the fish into 1/8 inch (5mm) slices. You can do this by slanting the knife to the right, starting from the left end.
Now your fish is prepared! You can serve the fish as is for sashimi, or prepare the rice for sushi.