If you are going into competitive battles, then you need more then just lv.100 Pokemon. You need strategy, skill, precision, good moves, thinking ahead of time, good IV's and EV-trained Pokemon.
IV's are Individual Values that determine your Pokemon's performance. You can better your IV's by continuously breeding Pokemon with good IV and passing them down to the egg Pokemon.
EV's are effort values that can be obtained through battling other Pokemon. Every Pokemon gives different EV values. For example: battling and defeating an Alakazam gives you 3 EV for special attack. You can omly max EV train a Pokemon till 510 EV at max. You can only max EV train a Pokemon on 2 stats. These 2 stats can't surpass 255.
Natures strongly affect your Pokemon. If you are running an Adamant Alakazam it's like putting fire beatles on a peanut butter and jelly sandwich. A recommend nature for an Alakazam would be modest or timid. Modest raises SpAtk. but lowers it's attack stat. Timid ups Alakazam's speed but lowers it's Attack stat. Alakazam is a whimp when it comes to physically attacking Pokemon but is a veteran at Specially attacking Pokemon. For this reason it really doesn't matter if a nature lowers it's attack stat.
Good moves determine a whole battle. For example using 4 psychic on Alakazam is useless. It is recommend that you use different type of moves on a Pokemon. This helps it counter or have coverage on it's weaknesses.
Building a good team is very important. You should build a team where every Pokemon supports and can work well with it's teammates. In the competitive world this is often stated as synergy.
Items determine a whole lot.for example: A choice band on an Alakazam is like filling your pants with fire ants, in other words useless. Choice specs isn't bad but still not recommeded. Life orb on magic guard Alakazam is probably the best combination for Alakazam.
This is just a short introduction. There are many great battlers on this site for example : Hex, AlphaDraconis, Leboss, |SentByRavens|, Dr.Flame (but he is kind of retired), Riles, Enoch, Breaking Point, Flare, Master Kijani, Poke'Slash, Flafpert, themadradlillypad, trachy, Mewderator, Ninja, Pokewatt, Mike, Pokemaster (but he is often very busy) and a lot more.
If you follow all these steps Alakazam will end up looking like this:
Alakazam@ Life orb
Trait: Magic guard
Nature: Timid/ Modest
EV spread: 252 SpAtk/ 252 Speed/ 4 HP
-Focus Blast/ Shadow ball
- Hidden Power ( Ice)
- Energy Ball
This covers a lot.
Hope I helped and welcome to the world of competitive battling :)