"Powinienem pomóc mojej babci ugotować obiad."

Translation: I should help my grandmother cook lunch.

[Wippii]

Being able to understand long sentences like this without any trouble feels great!

[Jack.Elliot]

babcia, babunia informal
My granny used to sing that song to me when I was little

[JoanieKatz]

Could I also use: "I should help my grandma?"

[Jellei]

"grandma" instead of "grandmother"? Yes, sure.

[Glen624972]

Is this correct? I would use "pomoć" if I mean this one time, but "pomagać" if I mean repeatedly?

[Jellei]

Actually, although more than 99% of infinitives end with -ć, "pomóc" is among this small fraction that ends with -c, so one needs to pay attention here ;)

Other than that, you are right. "pomóc" in some specific matter; "pomagać" in general, repeatedly.

[Glen624972]

Thank you, Jellei.

[Sarielowich]

Seems that to is missing here: I should help my grandmother TO cook lunch.

[JerryMcCarthy99]

"To" is not required in my variant of British English, although it's not wrong to use it. In any case, having "to" in your answer is accepted.

[edB4zK]

I think "powinienem" can also be past tense? "I should have helped my grandmother to cook lunch"?

[JerryMcCarthy99]

Well according to https://en.wiktionary.org/wiki/powinienem, "powinienem" (no infinitive form) is already in the past tense form.

The further tense back in time is the pluperfect which would be: powinienem był / powinnam była.

[Jellei]

I strongly recommend using pluperfect when applicable, but technically "powinienem" itself can have the "I should have helped" meaning. I just added this.

[edB4zK]

I had not encountered "powinienem był", thank you very much for your help :)