きょう

Your sentence can be understood as several meanings and I'll write some of them. 

(Today, my lovely cat ate fish.) *This fish is for her/his food. 

きょう、私のかわいい猫が魚を食べてしまいました。

(Today, my lovely cat ate my fish.) *This fish was also your pet or it was your food, the one that was not for your cat.

いま、私のかわいい猫が魚を食べています。

(Now my lovely cat is eating fish.) *This fish is his/her food in my opinion.

いまから、私のかわいい猫が魚を食べます。

(From now, my lovely cat will eat fish.) *This fish is his/her food in my opinion. In many cases, the present form is used for future in Japanese.

I hope this was helpful. If all of them aren't what you wanted to say, please tell me. 

きょうは。。。

I'm not 100% that this is the best way to say it, but 食べます would imply that the cat has yet to eat the fish. 食べてしまいました is the past tense wa to say "(unfortunately / regretfully) my cute cat ate a fish".