Is it me, or my Pokémon?

[Minokun]

I just recently got into competitive battling, and I'm not very good at it already... I was wondering if it was my team that was messing me up or just my lack of knowledge on this new field of Pokémon battling. Don't get me wrong, I know the basics of competitive battling it's just that I don't know the really technical stuff... Here's a team I created it's won some, but lost a lot!

Tyranitar(Mega)
252Atk/252Spe/4HP
Nature: Adamant
Ability: Sand Stream
Moves:
Stealth Rock
Stone Edge
Superpower
Crunch

Starmie
252SpA/252Spe/4SpD
Nature: Timid
Item: Leftovers
Ability: Natural Cure
Moves:
Surf
Thunderbolt
Ice beam
Rapid Spin

Aeigislash
252Atk/252HP/4Def
Nature: Adamant
Item: Weakness Policy
Ability: Stance Change
Moves
King's Shield
Shadow Sneak
Sacred Sword
Iron Head

Garchomp
252Atk/252Spe/4SpA
Nature: Naive
Item: Choice Band
Ability: Rough Skin/Sand Veil (Can't decide)
Moves:
Outrage
Earthquake
Fire Blast
Dragon Claw

Aggron(Debatable)
252Atk/252Spe/4HP
Nature: Adamant
Item: Life Orb
Ability: Rock Head
Moves:
Head Smash
Rock Polish
Iron Head
Earthquake

Espeon
252SpA/252Spe/4HP
Nature: Timid
Item: Life Orb
Ability: Magic Bounce
Moves:
Psychic
Hidden Power Fire
Grass Knot
Dazzling Gleam

Please Tell what you think! Is it me, or my team?

 

[PlatinumDude]

Regular Tyranitar is better off performing Stealth Rock support because it has passive recovery in Leftovers. Mega Tyranitar, on the other hand, is best played as a Dragon Dance sweeper because it has the Speed to take advantage of the Speed boost Dragon Dance provides:
-Crunch
-Stone Edge/Ice Beam
-Earthquake
-Stealth Rock/Fire Blast
Nature: Impish/Relaxed
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Sand Stream

or
-Dragon Dance
-Stone Edge/Crunch
-Ice Punch/Crunch
-Earthquake
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Tyranitarite
Ability: Sand Stream

Physically-based Aegislash is crippled by burn very easily. It's best off as mixed, since its Blade forme has equally high offenses and Shadow Ball is stronger than Shadow Claw:
-Shadow Ball
-Shadow Sneak/Sacred Sword
-Iron Head/Sacred Sword/Pursuit
-King's Shield
Nature: Quiet
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers

or
-Substitute
-Toxic
-King's Shield
-Shadow Ball
Nature: Modest
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers

or
-Shadow Ball
-Shadow Sneak
-Flash Cannon/Sacred Sword
-Pursuit/Sacred Sword
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb/Spooky Plate/Spell Tag

or
-Swords Dance
-Shadow Sneak
-Iron Head/Shadow Claw
-Sacred Sword/Head Smash
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb

Aggron is only good in its Mega form. In its regular form, Aggron is crippled by its common weaknesses to Water, Fighting and Ground. Since Aegislash tends to draw in Fire and Ground moves, Dragonite is a viable substitute:
-Dragon Dance
-Outrage/Dragon Claw
-Earthquake
-Extreme Speed
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Lum Berry/Weakness Policy
Ability: Multiscale

or
-Outrage
-Earthquake
-Extreme Speed
-Fire Punch/Dragon Claw
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band
Ability: Multiscale

or
-Dragon Claw
-Earthquake
-Extreme Speed
-Roost
Nature: Adamant
EVs: 216 HP/64 Atk/228 SDef
Item: Leftovers
Ability: Multiscale

 

[Minokun]

So all of my other Pokémon are fine? A lot of people have suggested switching Espeon and Starmie but never told me why, or what to switch them to.