It depends on whether the adjective is an い adjective or a な adjective.

い adjectives (which end in い, like おもしろい) must drop the い and add かった for the past tense (and use です). So おもしろい becomes おもしろかった.

On the other hand, な adjectives don't change, but use でした instead of です to signal past tense.

Note that な adjectives are adjectives that aren't い adjectives, but some な adjectives end in い, just like い adjectives, so you'll just need to memorise which is which, really.

No.