whichever one is the most difficult for you to get is the one that you should focus on. and that depends entirely on your preferences and habits. if you love chicken and could have 3-4 oz at every meal or snack, then focusing on protein might not be the best value for your efforts, particularly if you don't like a lot of vegetables. if you could eat any veggie, any time, any prep then but dislike meat, then focusing on protein is going to give you the highest return for your effort.
start by tracking a few days doing just what you usually do. once you see the total breakdown of what you're eating versus what you should be eating in terms of protein, carbs and fat you can make the decision to work on the macro that's most off. once you get the macros in line you can move on to other nutrients.
-google first. ask questions later.