I've made an revolver + sprites and sounds, it's pretty much finished, all that it needs is something that makes it stand out. I noticed that hitting the head of an enemy generally instagibs him, while shooting the body or legs deals about 20~30 damage. (Plus the damage of impact when the enemy gets knocked backwards and flies into a wall, but that's fun and I won't fix it) I was wondering if it was possible to have a large text pop up above an enemy such as CRITICAL HIT! when you headshot an enemy. Is there any way to do it? I'm new at modding and I've never used LUA before.

We don't have access to attachables yet, so we can't access the burst damage for wounds, nor the emission damage. Also, we don't have drawing functions. When we have those two, it would be possible to write the damage to the screen.
If you want, I think right now you could make a "Critical Hit" thing by using the MOPixel's Destroy() or Remove() event (if those exist, not sure) and spawning a short-lived MOSParticle above the hit.

Ah, alright then. I do remember a mod (mercenaries mod I think?) having a text float above the soldier after reloading a weapon or something like that. I'll look into it.

If you mean the rail gun, it's gibbing a glow with a sprite that says 'Ready to Fire' or some such.

You could make a custom enemy who did the same thing when his head explodes, but you can't do it with lua just yet.

Also to do it on wounds you'd have to have custom actors which bleed a sprite with a muzzleflash that displays text.