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.