Place TNT on the ground
Ignite it with flint and steel
Ignite it with a fire charge
Power it with redstone
Activate it with a lever, button, or pressure plate
Light it with another explosion
Shoot it with a flaming arrow
Use lava or fire to ignite it
Use a dispenser with flint and steel or fire charge
Punch it after it is primed to move it into position before it explodes