What supplements should I take?!

Exercise. I know, that's the obvious answer, but I wanted to preface with that first... without exercise, no supplement will make you gain muscle.
As far as supplements to take with exercise, any type of protein will be good. It doesn't matter if it's any specific brand of Whey or Soy protein or a lean cut of steak or chicken... any protein is what's needed to build muscles. Biologically speaking, when you exercise, your muscles are broken down and your body needs protein in order to rebuild them... if you don't have enough protein, your muscles will take longer to heal and your workouts won't be as productive in the long run.
If you feel sleepy after a workout, you may be using up all of your energy and don't have enough nourishment in general... you could try eating some carbs a good hour or so before you start working out and eat the protein after your workout.

With all that being said, who says you only have to focus on just one thing? I'd look at what helps you the most. L-Arginine is a great option because it's a simple Amino Acid (a building block of proteins) that helps circulation so it helps in the warm up and helps to prevent Lactic Acid from building up (that build-up is why the muscles hurt), plus it's almost impossible to overdose on it... you'll have explosive diarrhea first if you even tried. ;-)

There's another category for energy products... Ribose, L-Carnitine, and Creatine are major players there. Any simple sugars (even honey, syrup, or table sugar) can give you a quick boost, but it's followed by a quick crash... Ribose and Creatine help more for sustained energy to keep the workout going. L-Carnitine is an Amino Acid that helps your body convert fat into energy so many people use it for exercise and/or weight loss.

You may choose a few of these for various reasons or you may opt for none of them... that's an individual choice. I'm not a body builder so I don't use these every day (I do exercise on a regular basis, though), but I make sure I have some protein source after I work out and I take some L-Arginine if I have an extra intense workout or if I feel I'm going to be sore the next day.

As for the last question about focus and mental clarity, that's going to depend more on your body's response than anything else. Some of the most popular and beneficial nutrients for brain health are Ginkgo Biloba, Acetyl L-Carnitine, Fish Oil (Omega 3), Phosphatidylserine (abbreviated PS), DMAE, Huperzine A, Bacopa Monnieri (or Monniera), and Vinpocetine.
There are a LOT of people that get a lot of benefit from any combination of those. I would suggest adding one at a time to your daily regiment as too much too fast can be tough. I would recommend starting pretty much in order of how I listed them... that's a pretty good listing in order of popularity and effect for the general population. Good luck and I hope I helped!
Good luck and I hope I helped!

Health/nutrition classes, personal experience, and personal research in my role as a Product Specialist for a vitamin company

I have to say that the best supplement that I have found is an Omega Complex. At least Omega 3 but Omega 3-6-9 is best. We simply don't get enough fish in our diets and these essential fatty acids improve cognitive function and overall health in my experience.

One caution: Make absolutely CERTAIN you get "molecularly distilled" Omega Complex. This removes the heavy metals like mercury in the fish oil.

Rhodiola is the perfect thing for that and is organic

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Supplements are not the best way to go, because:

1) They have no taste.
2) They work on the placebo effect.

So long as you believe in the supplement, you can take any one you want for anything and it will work. Once you know this fact, none of them will work.

Supplements are nothing but food.

The way the body works, is that it makes you crave tastes of specific foods at specific times, depending on what nutrients your body needs at any particular time.

So eat a variety of foods, so your body can store a good internal database, and then just eat what your body craves.

This is why no one tends to want to eat the same exact meal everyday, because your body needs other nutrients than just those found in that one type of dish. So depending on what your body is low on, it makes you hungry for specific foods and tastes.

Trust your body, it knows more than any nutritionist, and there is never any general rule that fits every individual.

What your body may need today, may not be what it needs tomorrow.

Since when you swallow a supplement in a gel capsule, you can't taste it, your body has no way to keep track of that supplement and make you CRAVE a taste for it the next time your body may want or need it.

Having to ask another person what supplement you need right now, is like asking another person how thirsty you are right now and if you should drink water or not.

Humans survived and THRIVED for billions of years on this basic system of eating things by taste, without ever knowing what vitamins or supplements were, or having any nutritionists tell them what to eat.

As people get older, they find they also don't have tastes for the same foods they did when they were younger, and like foods that they disliked as kids, because our bodies need different nutrients as they are older than when they were younger.

Pregnant women crave different unusual food combinations, because their bodies need different nutrients to create the fetus inside of them.

Nature will give you all the information you need, just eat what you crave.