Learn English – Assigned to a job I know nothing about it – is there an idiom for that idea

Your first example, where the sentence is technically true but conveys a false meaning, would be considered sophistry in English. Informally, we might call these weasel words or, in some situations, use the recently coined word truthiness.

If you tell the truth about something, but hold something back, as in your second example, you could call that a half-truth. If you say, "I can't be drunk, I only had one beer," but you also drank a bottle of vodka, that would be a half-truth. This seems closer to your second example.

However, in my experience, most English speakers would be more likely to describe these situations in more general terms, as a trick, than to use a term specific to this sort of language.

"Get the basics straight" and "Get the basics straight first" are valid English, but are not idioms. In a context where an author is trying to:

• Be polite (perhaps sarcastic, but neither rude nor obscene nor profane)
• Point out the basics of something, before pointing out more advanced features
• Write clearly
• Use the second-person imperative

"Get the basics straight" or "Get the basics straight first" is a natural thing to say.

Here are some alternatives that are idiomatic. Notice that some of these alternatives are quite rude, or compare the reader to a baby, toddler, or young child:

• You have to crawl before you can walk.
• You need to walk before you can run.
• Learn the ABCs of <topic>
• Get your shit straight.

Here is an idiomatic alternative that is polite:

• Start with the basics.