Learn Dutch: We accepteren het resultaat helemaal niet. - We do not accept the result at all.

Because this sentence has a specific direct object: het resultaat (the result). In Dutch, when you negate an action that involves a specific object, niet is placed after that object.

What does helemaal add to this sentence?

By itself, helemaal means completely or entirely. When placed right before niet, it creates the phrase helemaal niet, which translates to not at all. They act as a pair and sit together at the end of the sentence.

Could I start this sentence with wij instead of we?

Yes! We is the standard, unstressed word for we. You would use wij if you wanted to put strong emphasis on who is doing the action, like saying WE do not accept the result (but maybe someone else does).

How do I know whether to use de or het before resultaat?

Resultaat is a neuter noun, so it always takes het. Because there is no simple rule for guessing which words are neuter, it is best to memorize the article together with the noun as het resultaat.