Basics: Which supplements should I take?

I have been asked many times, and can't begin to count how many times I've seen these types of questions online - "What supplements should I take? What supplements will make me lose weight? What supplements will make me gain muscle?" 
Truth is, there are no magic pills, or powders, that will do any of these things. Why take them at all then? First, let's look at the definition of supplement to help clear this up. 
"A supplement is something that completes or enhances something else when added to it."
The key to that definition is the something else. In this case, we're talking about diet and exercise.  Whether you're trying to get big, or trying to get lean - if you're not busting your ass at the gym, and taking your nutrition seriously, you won't get the results you want. Period
One of the most overhyped, overbought supplements I would say are fat burners. Time and time again people buy these, and then complain about the results. Then people will ask me -  "Have you tried this fat burner? I didn't get the results I wanted - which one is the best one?" Do they really work? Yes. Are they a magic pill that will help you lose weight? No. You can take these everyday and have zero results if nothing else in your regimen has changed. They do contain numerous ingredients that are proven to aid in weight loss - but they are only an aid. If you're not putting in the work in the weight room, and eating at a caloric deficit, guess what?! You just wasted, minimum, $30!

Great, so now I'm going to the gym, and have a solid diet in check - which ones work? There's so many. To just speak on a few I personally would say, take a look at Protein Powder, Creatine, and Pre-Workouts.
One of your three basic macros, this is the nutrition king. This is what's going to help you build muscle, recover, and stay full. Remember though, this powder is a supplement. You can't drink protein shakes all day - well you can, but eat food. How much should you take? That depends on who you are and what your goals are. Here's a basic example to help you figure it out. When training, you really should be consuming about 1g of protein, per 1lb you want to weigh. If your goal is to weigh 180lbs - consume 180grams of protein per day...easy.  How much is from food, how much is protein powder? Typically a 50/50 split will do just fine. If three of your food meals per day consist of 30g each, and your protein powder has 25-30g/ serving - you need three of those too to meet your goal. This is why protein powder is such a great supplement. You ate 3 great meals that day, didn't reach your macro goal but can't fit in too many calories without being a fatty - now you can reach it and only consume about 100-150 calories depending on your particular protein supplement. 
I remember growing up thinking Creatine was scary. I heard Mark McGuire was on this stuff. He was a freak monster in a race for the most home runs in the league, with forearms bigger than my head. This is something meatheads took to get big, but at the cost of getting a bloated look you would have to lose later on, right?  This is not the case, Creatine just got a bad rap over the years. It is a naturally occurring nitrogenous acid in your body that supplies energy to the cells in your muscle.  There are two schools of thought. Take Creatine before working out vs taking it after. Both are right - I prefer the latter. Taking it before can give you an added boost in your lifts by having more on tap to fuel you through your workout. Taking it after, will help replenish what was lost during your workout. I personally take it after, since my pre workout has some in it already, and I'm big on recovery so I prefer to supplement with it after I lift since my muscles are then depleted and could use the boost. Reality? Both are wishful thinking, and timing may be irrelevant to be honest - any time of the day will work, but if you feel better taking it pre or post - roll with it, can't hurt.
This is everyone's favorite as far as I'm concerned. For those not in the know- this is a powder packed full of different ingredients to help you get a better workout. It usually tastes good, and effects are noticed within 20-30 minutes. Typical ingredients like caffeine and beta-alanine are those you can feel work and truly up the intensity of your workout.  If you're lazy, going through the motions, and bullshitting around the gym, face it - you're wasting your time. A good pre-workout can help keep you focused, and give you that extra boost to push you to your limits. Be smart. Don't take too much and give yourself a heart attack. If you're building a tolerance and need more than 2 scoops, lay off for a while. That's not a bad thing. It's good practice to use your own will to push yourself... and after a week or so you can add it back into your routine and enjoy the benefits. 

In conclusion, as the header on this page says - interested in fitness? Be a student, educate yourself.  there are no magic pills to get your results. Don't ask me, or the Internet what to take if your diet is not on point and you're not regularly going to the gym. Once you've got all that sorted, do your research and see what you really think is worth your money.