Well, let me be the first to say that with Smogon, they have alot of helpful articles and threads that will help you get into Competitive battling, though they generally don't really "reteach" the basics of EV/IV training.
Seeing as I beat most of the other people to this, I'll throw in a few sentences to help you out, and well, I'll let the sages read and see if they agree with my lowly knowledge.
Part 1: EV accumulation
If you want to raise a stat, let's say ATk for Tangrowth, you have to battle a Pokemon like Bibarel or Chatot. But in order to get the extra number in that stat you have to battle four of Pokemon who have Atk ev's? I don't get it. There are Pokemon like Bibarel who have 2 ev's.
When you are raising a Pokemon's EV's you have it right so far. You want to raise attack, so you fight Pokemon that give up the EV in attack. Now, as far as that 4 point=1 stat point deal goes..you will not see that stat increase in full until you level up the Pokemon to lvl 100. Let me use your example.
Your Tangrowth is on lvl 1. You start having it attack Bibariels, and it gains Atk Ev's. You reach your 126th Bibarel (making you have 252 EV's in attack. You can have up to 255 EV's in any one stat [and 510 EV's total on any one pokemon], but 255 is not divisible by 4, so that's why EV spreads for Pokemon stop at 252-to make the highest amount of EV's that's divisible by 4 to have the best possible stat distribution.) and waste it. Yay, you now reached a maxed out Attack stat (EV wise) for your Tangrowth. You then begin simply leveling it up.
Your Tangrowth's Atk-during each level, will start to go up by much higher rates than if you did not train it (+3,+4, +6 +8, every level). By the time you reach 100-due to the way the math works, it evens out to 4 ev's to raise your ATK stat total by one digit. Still confused? No problem.
Here's another way to look at it. Let's say you have two Tangrowth's at lvl 50, Atk stat's are the same, at 124. No atk ev's gained on either one.
You kill off two Bibariels with one Tangrowth, and not with the other. When those two Pokemon reach 100, the one that killed off the 2 Bibariels will have 1 point higher in their attack total than the one that did not. (so one might be 250, and the other will be at 249).
Part 2: Training
#2) Also, how do you know how to train your Pokemon according to stats, natures and such? I mean, how do you decide which stat to increase and what nature is right? By choosing what role Your Pokemon will play on your team. If its gonna be a tank, you want good HP and Defense. If its gonna be a Sweeper, it needs good offensive stats. The natures help you localize what stat is to be the best and what stat you don't really have to worry about.
I know there's always smogon and i've read a lot of pages over the past few days there, but i can't help but wonder why natures that increase the same stat as others are ignored.
Because they decrease another stat that does not benefit the Pokemon. Ex. A Timid Jolteon is preferred over other Natures that raise Speed because Timid raises Speed but drops Atk. Jolteon doesn't really use its Attack stat, so that benefits the player by not having to waste EV's on it.
I have a Roserade with a Sassy nature and one of the smogon suggestions is a Calm nature Roserade. I get that speed is important but wouldn't ev training to increase that stat help? Yes and no. In this case, you would be working against yourself, since the nature cuts back on how well you are able to accumulate EV totals for Speed.
Quick Side trip. With natures, you get a boost and a defecit in your training. The stat that is boosted is increased by a multiplier of 1.1, while the stat that is dropped is multiplied by a value of 0.9. So even if you EV trained a Roserade with Sassy, you would need to spend MORE Ev's trying to have it reach a speed that a Calm Mind Roserade would reach in less time because its nature is not hindering it in that regard.
Part 3: Uniqueness
#3 I just wouldn't mind not relying 100% on smogon and trying to make my own strategies. The Strategies on Smogon are the collective work of folks that have played and studied the metagame for years. New strategies always come up and surprise them, and that's what makes the game dynamic.
You can rely on their sets, and you can experiment and make your own. They just are used alot because these sets (at least the standard ones) are tried and true (depending on the metagame). So feel free to think outside the box-they're there for your reference at best-is what I can say.
Part 4: Revision
Secondly, if your Pokemon has got the effort ribbon, if you try to reduce some unwanted ev's with the appropriate berry, would it work? And is there a way to find on what stat the ev's went, if you have messed up in training? I guess i know the answer on this... XD
Yes you can reduce your Evs after you get the Effort Ribbon.
As for messing uy in training, well that's a bit difficult without clearing out all the ev's (alot of berries). I usually write down what I off so I notice if I make a weird mistake somewhere along the way.
x_X Wow that was long