Minecraft wiki entry for Multiplayer_PvP_bases, associated text: You have a ton of dirt, but that's nowhere near good enough for a wall. At a minimum, you want multiple layers of cobblestone.
Now's a good opportunity to start a shaft mine or even a quarry. If you want a self-building wall, make sure to get at least 4 stacks of redstone and a couple of stacks of iron to build the pistons and wiring necessary.
Obsidian may take a long time to obtain, but it will be worth it. If you don't have diamond tools you can use buckets of lava and pour water on them where you want them to be placed in a technique called molding. If you have the area claimed, obsidian walls are practically impenetrable through the use of TNT or trying to dig through, (with the exception of withers, if the server has not disabled them) unless a plugin on the server has changed it.
Another trick for your walls is to put sand or gravel inside them. This will make them somewhat self-repairing as if someone attempts to blow a hole in your wall, or dig through it, the sand or gravel will keep falling down and replacing the ones they break. It won't stop them, but it might delay them long enough for you to scramble to activate your defenses.
Try to decide whether to use obsidian or cobblestone for the defenses. Obsidian is stronger, but cobble is easier to obtain, so you can get more layers. Also, most servers will have a plugin that allows obsidian to be broken by TNT. (4-5 is the general amount it needs.)
A better way to make your base a bit more TNT-proof is to add a shield of either water or lava surrounding your base, as TNT doesn't damage nearby blocks when submerged in either liquid. On the other hand, watch out for those hybrid cannons.
It is possible to make regenerating cobblestone walls, using lava and water to make an infinite cobblestone generator in the shape of a wall. Hybrid cannons, however, can breach these kinds of walls.