How do i make Tnt Have more explosion damage in minecraft?

Well, by "more explosion damage" you might mean one of two things;

• Increase the explosion's radius, or
• Increase the explosion's power

Although you can't increase the radius on a single block of TNT, you could simply detonate multiple TNTs one after the other.

As for increasing the power, you can't (at least, not in vanilla Minecraft). You see, every time something explodes, Minecraft has a number assigned which is it's power. This number is constant with each event, but may be different between events.

(i.e.: every creeper will explode with the same power [3], but that does not necessarily mean TNT will have the same power[4].)

Now, you're probably thinking "Oh, I'll detonate multiple TNTs at once." The problem is, each explosion is calculated independently of each other, and damage to a block is not cumulative.

For example:

Sandstone has a blast resistance of 4, therefore TNT can destroy it since its power (4) is greater than or equal to sandstone's blast resistance. But, a creeper cannot ever destroy sandstone since its power (3) is less than the blast resistance. Since explosions are not cumulative, no amount of creeper explosions will destroy it.

Source: http://minecraft.gamepedia.com/Explosion

TNT's power cannot be changed in vanilla Minecraft. When TNT explodes the explosion damage will not stack with the explosion damage of other explosives exploding at the same time. Also there is no ExplosionRadius or ExplosionPower NBT tags for primed TNT. If you are a map maker and want more powerful TNT explosions, it can be done by replacing exploding primed TNT with creepers with a set explosion radius with 0 fuse.

More Power In TNT (Creeper) Via command blocks:

Setup:

/scoreboard objectives add noFuse dummy

Fill Clock:

/scoreboard players set @e[type=PrimedTnt] noFuse 1 {Fuse:0} //You can add more restrictions to the selector to only power up certain tnt
/execute @e[type=PrimedTnt,score_noFuse_min=1] summon Creeper ~ ~-1 ~ {Fuse:0,ExplosionRadius:<Explosion Radius>, ActiveEffects:[{id:14,Amplifier:1,Duration:10,ShowParticles:0}]}
/kill @e[type=PrimedTnt,score_noFuse_min=1] 

/summon creeper ~ ~ 5 ~ {ExplosionPower:3}