1. In Japanese, there is no word for "A" or "THE"
So, A FISH is SAKANA and THE FISH is SAKANA

2. The Japanese word for IS is DESU (DEH SOO)
Therefore, THE FISH IS is SAKANA WA DESU

Notice the word WA after SAKANA. WA after a word shows it is the subject of a sentence.

To say THE FISH IS RED you simply say, SAKANA WA AKAI DESU

Notice too that IS - DESU goes to the end of the sentence.