TNT can be lit by fire.
![TNT on fire][1]

TNT ignited by fire
![TNT ignited by fire][2]


Lava also causes TNT to ignite as the TNT catches fire because of the Lava. So does lightning if it strikes your TNT, it also causes the TNT to catch fire and it will eventually ignite.

TNT can also be ignited by a Redstone Pulse send out by a Redstone Torch, Button, Lever, Pressure Plate or Detector Rail. This is also the most common and safest as it doesn't accidentally light up wood near you.
![TNT ignited by Redstone pulse][3]


  [1]: https://i.sstatic.net/9BNls.png
  [2]: https://i.sstatic.net/DnpZ9.png
  [3]: https://i.sstatic.net/bmNyz.png