"I would rather not do it."
Translation:Jeg vil helst ikke gjøre det.
Helst is the superlative form of gjerne, while heller is the comparative form.
Whenever comparing two things ("I would rather do X than X") you would use "heller X enn X", or leave out the "enn X/than X" part if what you're comparing it to is implicit.
It's more than a little bit tricky to translate this adverb to English, so if you're not getting the difference between "heller" and "helst" it's probably because it's difficult for us to represent it in the English translations.
- Gjerne can sometimes be translated to "gladly", but other times it has no good translation.
- Heller is a direct translation of "rather" in examples like the one just given, but can also mean "neither" when paired with a negation ("Me neither" = "Ikke jeg heller").
- Helst lacks a good counterpart in English, and is usually translated as either "rather" (meaning you miss out on the "heller/helst" distinction), or rewritten with some variation of either "the most" or the verb "prefer".
"Jeg vil heller ha kylling enn fisk, men (aller) helst vil jeg ha iskrem."
"I would rather like chicken than fish, but I would (still) prefer ice cream."
"I would rather like chicken than fish, but most of all I'd like ice cream."
The first translation is the more elegant one, while the second one is a truer representation of the Norwegian sentence.