"make a meal" "cook a meal" "cook food" "prepare a meal" "prepare food" are all OK. "make food" does not work because food already exists. Speaking naturally, people often say sentences like : "I need to make / cook dinner soon" [a specific meal] "prepare a meal" is more formal and less common in speech. Perhaps a teacher from the US could talk about the usage of "fix" e.g. "fix a snack", which I understand is used informally. I don't think "fix" is used in place of "cook" for a main meal, but others would have to confirm. There is some UK slang too.

Yes, you can say (in the US, informally) that you are fixing dinner. You wouldn't say you are cooking dinner if it's a salad, though, and "making" or indeed "fixing" would be better in that case. "Preparing food" would cover it too, but to me that has the connotation of being disconnected from the meal at which it is to be consumed, or of not constituting the entire meal: you might say "I am going to make/cook dinner" in the expectation that you will proceed to eat it, whereas you might talk about "preparing food" let's say for a party or a picnic later.