Firstly; this is intended to help you cover your bases. None of these recommendations are meant to replace real food in your day to day life. The ideal scenario would be you not needing any of these things. Sadly, this just isn't likely to be any of our realities. Before starting to supplement things in your diet obviously consult your doctor. There could be contraindications with medications you're taking that may have undesirable results. I'd also highly recommend getting a full blood panel done as well. You can't fix what's going on until you take a peak under the hood every once in a while.
Ok information time: I'm going to break this list into 2 sections. Health and Performance. There's obviously a made up solution for every made up problem to sell you more stuff so I'm just choosing the most basic of options with the most research backing their efficacy.
Fish Oil: There's basically no downside to taking this. Omega 3's help with basically every cell function. When shopping for this look for 2 things on the label; EHA and DHA totals. Theres are the essential fatty acid's we're after; aim to get 2 grams of those in per day. This is as confusing for me to type as it is for you to read. If you need help finding a reputable brand feel free to reach out and I'll be glad to help.
Vitamin D: Again many functions of the body rely on this. If you're not getting 30-45mins of sun during peak times (10AM-2PM) in daily aim to supplement this with at least 2,000IU's daily.
Magnesium: Magnesium is used in literally every cell function responsible for keeping you alive. It's good; take it. Find a chelated version as they're much easier on the stomach.(any variant ending with "ate" at the end of the word when reading the ingredient list on the back will do.)
Protein: You've all asked about this. Use protein powders to supplement your diet when food isn't easily accessible. If you're struggling to hit your daily recommended amount of protein (1g/lb of bodyweight) try supplementing this daily. Timing doesn't matter; protein consumption is a cumulative effect when intake is high enough regularly.
Creatine Monohydrate: Don't get wrapped up in the all fancy versions available as they're just expensive marketing trash. Don't support that shit. Creatine Monohydrate is the second most studied supplement on the planet behind caffeine (with a literal century of testing behind its efficacy). Aside from helping with tissue growth; it's also being show to help stave off brain tissue degradation in Alzheimer's patients in more recent studies. Take 5 grams per day for ever or we can't be friends.
Caffeine: Has been studied for centuries; you're probably already taking this daily. Obviously helps with focus, mental clarity, performance. Tl;dr keep the coffee flowing folks.
Sodium: Good old salt. If you're an active person you can easily be losing up to 2000mg of sodium through perspiration per HOUR of activity. The other side of the hydration coin is sodium intake. Have you ever experienced that lingering "over-tired feeling? Cramps, stitches, muscle pulls, stalled weight loss, stalled weight gain, stalled performance, hair thinning, hair falling out? You may be chronically dehydrated because you're missing key minerals that have been demonized by the wildly unaware.
I highly recommend picking up a copy of "The Salt Fix: Why the Experts Got It All Wrong--and How Eating More Might Save Your Life" by Dr. James DiNicolantonio for more info on the topic if you're interested.