Learn Russian: Им некого попросить о помощи, поэтому они часто жалуются. - They have no one to ask for help, so they often complain.

This is a special negative construction meaning 'there is no one to [do something]'. In Russian, the person experiencing the lack is the logical subject and must be in the Dative case (Им). It translates literally as 'To them, there is no one to ask.'

What is the difference between некого and никого?

You use некого (always followed by an infinitive) to show that there is absolutely nobody available for the action. If you used никого, you would need a regular conjugated negative verb, like Они никого не просят (They don't ask anyone).

Why does 'to ask for help' translate to попросить о помощи instead of using the preposition за?

While the English word 'for' is often translated as за, the verb попросить (to ask or request) specifically requires the preposition о + Prepositional case to express the thing you are asking for.

Why does the noun помощь change to помощи here?

помощь (help) is a feminine noun ending in a soft sign (a 3rd declension noun). After the preposition о, it must take the Prepositional case, which for these types of nouns ends in -и.