It is fine today.

We use "it" to describe the weather.
But in Japanese we don't have any word functioning "it" in above sentense.

tenki : weather
yoi / ii : good, fine
kyou : today

Kyou wa tenki ga ii. : today , weather, good. : It is fine today.

"wa" of "kyou wa" is meaning " in terms of " or " regarding", not "wa" which is attached after the subject.
Literaly meaning is " Regarding today, weather is good.".

ashita : tomorrow
ame : rain(noun)
ame ga furu : rain(verb)
furu : fall, drop

Ashita wa ame ga furu darou. : Tomorrow, rain, fall, will : It will rain tomorrow.