Cashwarrior's X Y battle team.

This is basically my team that I'm playing with, I will give the moves I currently have and the things I will be planning on having. Here's my team as of right now: (Just to let you know, I am a complete noob, I have no idea what EVs/IVs are.) I will presenting you with a full bio of my pokemon because I don't know what things you need to know to help me improve my team.

My team:
Braixen, "Ferrin" Lvl. 53 Adamant and Highly persistent
Moves:
Cut (I want to replace with magic room)
Psychic
Flamethrower
Sunny Day
Ability: Blaze (I want to change it)
Item: Eviolite
Stats:
Hp: 140
A: 92
D: 82
Sp. A: 99
Sp. D: 92
S: 95

Venusaur, "Bulst" Lvl. 52 Bold and Loves to eat
Moves:
Frenzy Plant
Toxic
Venoshock
Petal Blizzard
Ability: Overgrow (I want to change it)
Item: Black Sludge
Stats:
Hp: 162
A: 97
D: 113
Sp. A: 119
Sp. D: 122
S: 102

Avalugg, "Bep" Lvl. 52 Docile and Quick to flee
Moves:
Ice Fang
Ice Ball
Surf (Want to change for good water attack)
Recover (Want to change for Blizzard)
Ability: Ice Body
Item: Leftovers (Want to swap for Metronome because Ice ball is basically rollout and I want to see if it becomes super strong)
Stats:
Hp: 168
A: 144
D: 210
Sp. A: 60
Sp. D: 68
S: 54

Lapras, "Lirp" Lvl. 52 Docile and Takes plenty of siestas
Moves:
Thunderbolt
Body Slam
Ice Beam
Waterfall
Ability: Water Absorb
Item: Metronome (Want to swap for leftovers)
Stats:
Hp: 216
A: 110
D: 100
Sp. A: 110
Sp. D: 118
S: 86

Finally,
Marowak Lvl. 53 Hardy and hates to lose
Attacks:
Strength (Want to change for a stat changing move)
Bonemerang
Brick Break
Double-Edge
Ability: Rock Head
Item: Rocky Helmet
Stats:
Hp: 136
A: 102
D: 130
Sp. A: 70
Sp. D: 104
S: 67

What should my last pokemon be?

Please don't tell me how bad my team is, I'm literally just starting to get into competitive battling.

Well, for one, you can only use an ability capsule to change pokemon between two normal abilities (if they have more than one), not between normal and hidden- for your Braxian and Venusaur, you would need to breed new ones from a parent with that hidden ability (which you can find in the friend safari depending on the people in your friends list, after you beat the game.

As for EVs and IVs, I can help explain those if you want.. and natures, as those are important too.

For your current team- Get your Marowak its signature item. It's really only good with the thick club, as that doubles its attack.

Okay, I guess I'll have to search the world for a bone, thanks!

Ev's and Iv's are pretty important in competitive battling.
As for the team balance, I use this: http://www.teammagma.net/teambuilder/