So, I assume that you've checked out basic Lua tutorials and that the main issues are with ESO's API. If you let us know exactly what's confusing, might be able to give more targeted help.
If you know Lambda functions from Java then that's pretty much the same as Lua's functions.
You'll want to call EVENT_MANAGER:RegisterForEvent(stringIdentifier, eventConstant, functionToRunOnEvent). StringIdentifier is well anything, though it should be unique. Doesn't matter too much as long as it's unique tho. eventConstant, check out
https://wiki.esoui.com/Events, specifically probably
https://wiki.esoui.com/Events#Achievements. Finally, the function is the meat of it, it'll be run anytime the event happens. See the events page for what the parameters that get passed to it are.