Hey... In an attempt to get back into the game, I'm going to revise the team building guide some.
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First of all, here are some links regarding EVs. I HIGHLY recommend that you read both of these...

An In-Depth Explaination of EVs by Serebii.net
Fallen Ronin's "The Fine Art of Stat Manipulation

Also, on Serebii, look for "Individual Values" and read that article.

Once you have that over with, you should have a fair amount of knowledge. Email me any questions you have about these things after you've read them.

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Here are the Natures you will likely be using most through out your time playing the game:

Adamant: +Atk -SAtk
Brave: +Atk -Spd
Naughty: +Atk -SDef
Modest: +SAtk -Atk
Quiet: +SAtk -Spd
Timid: +Spd -Atk
Jolly: +Spd -SAtk
Bold: +Def -Atk
Impish: +Def -SAtk
Calm: +SDef -Atk

There are others, but you usually won't use them competitively.

Here are the types of attacks:

Physical attacks are attacks from the following types that deal damage: Normal, Fighting, Flying, Ghost, Rock, Ground, Bug, Poison.

Special Attacks are attacks from the from the following types that deal damage: Psychic, Electric, Ice, Fire, Grass, Water, Dragon.

Please inform me if I miss anything. I'm kinda in a hurry. lol

When you make your team, always keep in mind the role that your Pokémon will play. Whether it's healing your team, inflicting status ailments, or simply attacking full-on, it will have a certain way that it's used best, and some natures that will help it more than others.

Here are the natures for common roles in your team, along with a brief description of the role and an example of that Pokemon.

Physical Sweeper: A Pokémon that is mainly used to attack using physical attacks. Usually, when using a Pokemon of this type, you'd want a nature that increases Attack or Speed. Sometimes (rarely), you'll want a Defense or Special Defense Raising Nature. You would also want to pour EVs into Attack, Speed, HP, and/or the defenses.

Special Sweeper: Same as a Physical Sweeper, but with special Attacks. You'll want to use a nature that increases Special Attack or Speed, sometimes Defense or Special Defense. EVs go to Special Attack, Speed, HP, and/or the defenses.

Tank: A Pokémon that can take a lot of damage, or sustain heavy hits easily. It is usually some sort of wall. Skarmory and Blissy are considered some of the best walls, but have lost some power recently. I like Claydol for reasons I'll explain later, and Weezing, though I never use him. Tanks usually want a Defense or Special Defense Raising nature. You almost always want to put your EVs in HP, Special Defense, Defense, and sometimes enough speed to beat out some slower Pokémon. If your Pokémon is a Physical Wall/sponge, the EVs go in Def, and vice Versa for Special Walls/sponges.

Baton Passer: A Baton Passer is a Pokémon that uses Baton Pass to pass along stat increases gained through abilities or through moves such as Swords Dance or Calm Mind, or pass along moves such as Mean Look and such. They usually either have high Defence/HP, like Umbreon or have really high speed, like Ninjask. You aren't usually attacking with them, so use a Defense or Speed increasing Nature. Put EVs in HP, Defense, Special Defense, and/or Speed. If you plan on attacking with them, usually only use one attack, such as Hidden Power [Bug] on Ninjask. Umbreon NEVER attacks. I'll e-smack you if you ever try that.

HM Slave: A Pokémon that use used in your party simply to have four HMs. You don't use them in battle, you don't waste TMs on them, and you NEVER use them on NetBattle. I usually use two in my party, Linoone and Tropius. Together, they learn every HM except for Dive and Waterfall, which, coincidently, are the least used HM moves. Linoone also has the ability to pick up items after battles. I usually level him up to level 91 in Emerald, since he gets the best items at that point with the new system implemented, but in Ruby, it doesn't matter the level.

Ask me about others.