I read this as [Wela au] [i kēia lā] == [ state-of-being-hot I ] [today]. In more idiomatic English [I am hot] [today].
So the sentence has the form [[Verb Subject] [ADVerbial-phrase-of-time]].
"Wela" is a verb that describes a state, rather than an action.
"Au" (also "wau") is just the pronoun "I".
"Kēia lā" by itself would be "this day", but it's best to treat the phrase "i kēia lā" as a unit and memorize it as an adverb of time meaning "today".
Additions and corrections welcome.