The supplement industry depends on the feeling that you are missing something. For most people who eat reasonably, sleep enough and train consistently, you are not. Supplements sit at the very top of the priority list, after all of that, and the honest list of ones worth buying is short.
Creatine monohydrate
Creatine is among the most studied supplements in sport. Taken at around 3–5 g a day, it helps muscles produce short bursts of high-intensity effort, and over time tends to give a modest benefit to strength and training capacity. It does not need loading or cycling, and plain monohydrate is the form with the evidence behind it.
Expect a small rise in bodyweight in the first weeks, mostly water held in muscle. In healthy people it has a strong safety record. If you have kidney disease, check with your doctor before using it.
Protein powder
Protein powder is a food, not a performance enhancer. It is useful when it makes hitting a protein target easier, for example on busy days or for people who do not eat much meat. If you already hit your target with meals, it adds nothing. Choose a product that has been tested by an independent third party for what is in the tub.
Caffeine
Caffeine has fairly consistent evidence for modestly improving performance and reducing perceived effort. Coffee works as well as a pre-workout product. Sensitivity varies a great deal between people, and caffeine late in the day can disrupt sleep, which costs you more than the caffeine gives you.
Many pre-workout products combine high caffeine doses with other ingredients, sometimes in amounts that are not clearly disclosed. If you use one, know how much caffeine you are taking from all sources combined.
Vitamin D, and everything else
Vitamin D is worth supplementing if you are actually deficient, which is not uncommon in winter or for people who get little sun. The sensible route is to ask your doctor whether a blood test is warranted, rather than taking high doses on the assumption you need them.
Beyond these, most products—fat burners, testosterone boosters, most amino acid blends, detox products—have little or no good evidence, or rely on doses too small to matter. Spending that money on better food is almost always the better trade.
What to do this week
List what you currently take and what you spend on it each month. Keep what is on this short list and serves a clear purpose for you, and consider dropping the rest.