Oh dear. Bad advice errywhere. I will use this to try and illustrate the competitive mindset, though. Excuse my essay
If you're playing singles, you must know about entry hazards.
Spoiler
These are Stealth Rocks, Spikes and Toxic Spikes. They deal damage to your Pokemon when the Pokemon switch in.
Stealth Rocks in particular can be very nasty, as they do damage based on whether you are weak to rock or not! If you double-resist Rock, it's only a little over 3% to your health, and if you single resist it's only 6.25% damage to your health. If you are neutral to Rock types it jumps up to 12.5%. Single weak to rock and that's 25% of your HP gone, and double-weak means that's half your health off. Luckily Stealth Rocks do not layer, but they still make sure those with rock weaknesses are hardly used.
Spikes works on layers (how many times the opponent uses the move). 1 layer does 1/8 of the opponent's HP, 2 layers 1/6th, three layers 1/4th.
Toxic Spikes is also layer based. One layer poisons the opponent, two layers Toxic poisons.
They're not as prevalent as they used to be but they must be considered when making a team. Rapid Spin gets rid of SRs, Spikes and Toxic Spikes so if your team is a bit weak to them it may be useful
Switching is more seen in singles than any other format, as it can mean giving yourself an advantage. Forcing the opponent to switch is very useful. Moves like Roar and Whirlwind can do this as well as just having a Pokemon on the field that walls or could really hurt the opponent.
If you're playing doubles, forget stat boosting moves. Doubles are just too fast paced to be wasting a turn on setting up a buff like that. You can try Trick Room, Gravity and Tailwind, as they provide more significant changes than just +1 in a certain stat on one Pokemon.
If you want a weather team of either rain or sun, try to get one of the Dream World Pokemon. Namely Poliwag with Drizzle and Vulpix with Drought. Their abilities are basically that of Kyogre and Groudon... they summon rain and sun (respectively) on the turn they enter battle. Make sure to evolve the Poliwag into Politoed though or it won't get Drizzle.
Rain is the premier weather right now and very popular as a team strategy. This is because so many Pokemon have Swift Swim, and that water is a very good attacking type. So make sure there aren't too many water weaknesses in your team.
You want consistency in deciding on movesets. Some people have suggested moves that have really low accuracy (eg Hypnosis). They often end up unreliable however during that crucial moment...
Using a team based around Gravity can fix this however a Gravity team can be challenging to pull off in doubles as it requires a turn to set up.
Moves like Fire Blast and Thunder (120ish base power and 85 accuracy) can be okay if you're comfortable taking the risk to get extra power. I would decide whether you want to go power over accuracy though instead of having something like Fire Blast and Flamethrower on the same moveset, as this is redundant
I tend to fancy healing moves since Items are not allowed in tournaments.
I don't know what kind of odd tournaments you'd been in, but they are allowed. It's just a case of not having the two same items on two Pokemon. Item choice is very important. Here are some useful items...
Spoiler
- Life Orb increases the power of all your moves by 30% at the price of 10% of your HP each turn
- Leftovers is superior over Shell Bell. At the end of each turn the holder recovers 1/16 of their HP
- Air Balloon causes the held Pokemon to be immune from Ground-type moves and entry hazards, but when the Pokemon is hit with an attack it bursts and stops working
- Choice Band increases the holder's attack by 50% but restricts it to using the first attack until withdrawn
- Choice Scarf increases the holder's speed by 50% but restricts it to using the first attack until withdrawn
- Choice Specs increases the holder's special attack by 50% but restricts it to using the first attack until withdrawn
- (If you are using a Choice set only have attacking moves on the moveset)
- The type-boost Gems are insanely useful. One-use items that boost the power of a move that has typing that corresponds with the gem by 50%. Two deadly combos are Dragon Gem Draco Meteor and Flying Gem Acrobatics
- Eviolite when held by a Pokemon still able to evolve increases their defense and special defense by 50%
- Expert Belt boosts the holder's moves that are super effective against the opponent by 20%
- Flame Orb is good with the Guts ability (which I would use on Conkeldurr over Sheer Force). Flame Orb burns the holder at the end of every turn if they do not have a status already. This activates Guts, which will increase attack by 1.5x and negates the attack drop from burn
- Focus Sash acts like Sturdy: if the holder is hit by an attack on full HP that would knock them out, Focus Sash activates and instead leaves them on 1HP (Focus Sash disappears after this)
- Iron Ball helps weather inducers, as the holder's speed is reduced by 50%... but the slowest Pokemon's weather ability activates last, which will overwrite the opponent's weather
- Mental Herb is an interesting one. If the opponent's move infatuates, disables a move or is Taunt, Encore or Torment, Mental Herb heals the user from the effects of these moves. However it is one-time use
- Toxic Orb is the same deal as Flame Orb except your Pokemon is Toxic poisoned at the end of each turn. Seems pointless, but put it on a Pokemon that has Poison Heal ability and they heal HP every turn instead of losing it
- White Herb is also interesting... at the end of using a move, if that move lowered the holder's stats, White Herb restores the stat drops so your stats are back to normal. One-use item
- Don't use the type-boost items such as Charcoal, Magnet etc. They only increase the power of a certain type by 10%. There are better items out there that provide more of a boost
- If you are going to use Rocky Helmet use it on something that has an ability like Iron Barbs and Rough Skin. I wouldn't really recommend it tho
Natures are also vital, as they can boost one base stat by 10% and lower another base stat by 10%. They can give you a defensive or offensive edge over the opponent.
You also want to consider EV (Effort Value) training as this can also help your stats a lot.
There is a whole lot more to competitive Pokemon battling then what I've listed though so please PM if you want to know more