Rate my team

[Raveg64]

1. Garchomp

Ability: sand evasion

Nature: Adamant

1. Sandstorm
2.Dragon Rush
3. Dragon Claw
4. Earthquake

Item: Garchompite

2. Milotic

Ability: Competitive

Nature: Calm

1. Hydro pump
2. Ice Beam
3. Surf
4. Confuse Ray

Item: Splash plate

3. Tyranitar

Ability: Sand Stream

Nature: Adamant

1. Crunch
2. Stone Edge
3. Earthquake
4. Stealth Rock

Item: Hard Stone

4. Malamar

Ability: Contrary

Nature: Jolly

1. Superpower
2. Hypnosis
3. Night Slash
4. Psyco Cut

Item: Mind Plate

5. Zoroark

Ability: illusion

Nature: Lonely

1. Night Slash
2. Night Daze
3. Agility
4. Foul Play

Item: Black glasses

6. Serperior

Ability: Overgrown

Nature: Rash

1. Leaf Storm
2. Leech Seed
3. Leaf Blade
4. Giga Drain

Item: Miracle Seed

Please tell me what I should change.

 

[TerraTF]

I can tell that this is your first Competitive team just from the build of your team and due to the missing EVs but I'll try to help you as much as I can.

1. Garchomp

Ability: sand evasion

Nature: Adamant

1. Sandstorm
2.Dragon Rush
3. Dragon Claw
4. Earthquake

Item: Garchompite

Alright, Mega-Garchomp to start off. Solid choice for a mega considering it's probably one of the top 3 still active Mega Pokemon. First off is the ability. In standard Smogon play, Sand Veil falls under the Evasion clause meaning no moves, abilities, or items that boost evasiveness are allowed. I'll post a link to Smogon's base OU rules at the end of this post.

Next onto the nature and EVs. Adamant is definitely one of the two natures you'd want to run on the set you're trying to make. I personally would go with Jolly due to the higher Speed but Adamant is good if you go with an all physical set. Since you didn't provide the IVs I'm going to hope that you're running 4 HP / 252 Attack / 252 Speed. That tends to be the standard for Garchomp that are used to land a lot of damage.

Finally the moves for Garchomp. Sandstorm, Dragon Rush, Dragon Claw, and Earthquake. I like that you're trying to pull off some of Garchomp's STAB moves but I feel that Sandstorm and Dragon Rush should get the ax. You would probably be better off replacing those two moves with Crunch and Stone Edge to help give Garchomp some extra coverage.

2. Milotic

Ability: Competitive

Nature: Calm

1. Hydro pump
2. Ice Beam
3. Surf
4. Confuse Ray

Item: Splash plate

Next is Milotic. I don't really recommend running any of the plates Competitively unless you're playing Ubers with Arceus. The ability of Competitive really only helps in Double Battles when you can have the partner hit it with moves that lower attack. I would say go with Marvel Scale over Competitive.

Nature and EVs next. Calm is a decent ability as it ups special defense and lowers attack, but I would have went with Bold so that you could even out its defenses to a more manageable 95/86/125 after the 10% bonus from Bold kicks into defense. For the EV spread I would have went 252 HP / 196 Defense / 60 Special Attack. That helps to add some more to defense while fortifying its HP and giving its special moves a slightly high chance of doing more damage.

Moveset time. Hydro Pump, Ice Beam, Surf and Confuse Ray. Another instance of a double STAB move. I would be force to choose between Hydro Pump, Surf and then add in Scald due to the burning side effect. I would also replace the move you choose to get rid of with Dragon Pulse. There is also another choice of running Light Clay as Milotic's Item and using Milotic as a screener. If you went with the screener set I would go with a moveset of Ice Beam, Scald, Reflect, Light Screen.

3. Tyranitar

Ability: Sand Stream

Nature: Adamant

1. Crunch
2. Stone Edge
3. Earthquake
4. Stealth Rock

Item: Hard Stone

With Tyranitar as your main source of setting up Sandstorm its item definitely has to be the Smooth Rock to extend the length of Sandstorm by 3 turns.

Nature is fine and IVs should be 4 HP / 252 Attack / 252 Defense. Sand Storm will give Tyranitar a boost to its Special Defense due to its rock typing.

Moveset is good but I would try to find room for Pursuit.

4. Malamar

Ability: Contrary

Nature: Jolly

1. Superpower
2. Hypnosis
3. Night Slash
4. Psyco Cut

Item: Mind Plate

I really don't see much from help from Malamar on a Sandstorm team. Maybe replace it with something that could benefit from the sand being on the field. I would say something along the lines of Excadrill.

5. Zoroark

Ability: illusion

Nature: Lonely

1. Night Slash
2. Night Daze
3. Agility
4. Foul Play

Item: Black glasses

Zoroark would be another Pokemon I would replace due to lack of usage on a Sand team. Possibly change it out with something that could set up Stealth Rocks and Spikes like Rocky Helmet Ferrothorn or Forretress.

6. Serperior

Ability: Overgrown

Nature: Rash

1. Leaf Storm
2. Leech Seed
3. Leaf Blade
4. Giga Drain

Item: Miracle Seed

Serperior is another Pokemon that should be replaced for a Pokemon that could sweep in Sand. Possibly Aegislash or Bisharp to help counter Fairy types.

Please tell me what I should change.

Overall this is a decent team for a first competitive team. Here is the link I promised earlier in the post. Hopefully the changes I provided helps you out.

 

[Raveg64]

Okay so I should probably set up different teams?

 

[TerraTF]

I would say try out this team until you start to not like how it works. The best way to see what wrong with a team is to use the team and try and fix its kinks.