(This is my opinion, you also have a right to your opinion, so don't go crazy on me.)
Step 1: Primary Gun
I will have a breakdown on the guns
Snipers: +Range +Power -Defensive +Offensive Recommended: PSG1 or L96
Shotguns:-Range +Power +Close Range Defense+Close Range Offense Recommended: SPAS-12
Assualt Rifles: +Range +Power -Recoil +Offense Defense depends
Submachine Guns -Range +Power with rapid fire -Recoil +Defensive gun Recommended: AK74u or MP5K
LMGs:+Range +Power -Recoil -Reload Recommended: Stoner63
Note: By defensive and offense I mean if you started the gunfight or are defending from the gunfight.
Step 2: Secondary Gun
Pistols: Can be effective or terrible, depending how you use them. Recommended: C275 Full Auto or Python Speed Reloader
Launchers: These can be effective for non-flak jacket opponents, and enemy air support.(Easy Points) Recommended: M172 Law
Specials: Because these are so different, I am giving them their own section
Crossbow: This is great for stealth, because you don't appear on the radar, and you can get an instant kill with certain accuracy
Ballistic Knife: Hard to use, no iron sights. Aim above your target
Step 3: Perk 1
Lightweight Pro : Sprint Faster + No Falling Damage. You can take advantage for no falling damage, like escaping out of a window.
Flak Jacket Pro : Reduced Explosive Damage + Reduced Flame Damage. Great for capturing objectives in Dom. Helpful for Noob Tube and Grenade Spamming
Scavenger Pro: Take ammo from fallen enemies + More Starting Ammo. You can use this with a high-rate-of-fire submachine gun. Very helpful for high killstreaks.
Hardline Pro: 1 less kill required for killstreak + Can reroll care packages. You should use this with the care package. Rerolling for 5 killstreaks and below is recommended.
Ghost Pro: Remain hidden from spy planes + Avoid attack helicopters, no red diamond from chopper gunner and gunship. No name appears when aimed on. Use this class to stealth around and flank. Don't forget your supressor.
Step 4: Perk 2
Sleight of Hand Pro: Reload Faster + Aim Down Sights faster, except with sniper scopes.
Steady Aim Pro: Reduces Hipfire Spread + Aim Faster after Sprinting, Faster Knifing
Hardened Pro: Bullet Penetration + More Damage to Aircraft, No Flinching when Shot.
Scout Pro: Hold Breath Longer with snipers + Change weapons and plant equipment faster
Step 5: Perk 3
Marathon Pro: Sprint Farther + Unlimited Sprint
Second Chance Pro : Pull out a pistol before dying + Can be revived by teammates
Tactical Mask Pro: Immune to Nova Gas + Flashbangs and other stun grenades stun reduced, can see stunned enemies on radar.
Ninja Pro: Move around almost silently + Move silently
Step 6: Equipment and Grenades
Equipment: Claymore: Effective for defending a room or objective. Counter=Flak Jacket
C4: Basically a throwable, user controlled claymore. You get 2. Counter=Flak Jacket
Motion Sensor: A small device that you plant in a certain area, like an objective,and see enemies that walk around in the radius.Counter=Hacker Pro
Camera Spike: A thermal camera that takes up your minimap. It should be used to watch an objective or bomb site.
No Counter that I have Found
Tactical Insertion: A counter to bad spawning or spawn trapping. Controlling where you spawn. Counter=The amount of light and sound it makes.
Frag: A throwable grenade that has a 5 second fuse. Can be thrown back.
Semtex: A grenade that has a 1 second fuse after sticking to an object.
Tomahawk: A knife that can be thrown. If hit by it, it is an instant kill.
Flashbang: A grenade that stuns your oppenent for about 1-4 seconds.
Willy Pete: A smoke grenade
Nova Gas: A gas grenade that can eventually kill you
Concussion: A grenade that stuns and slows you for about 1-4 seconds
Step 7: Put It All Together
1. Choose a type of gun you find comfortable, and attachments
2. Choose your secondary. Do you want to shoot down aircraft, or stick with crossbow bolts?
3. Perk 1 is your skin. Choose what you want the most.
4. Perk 2 helps with your guns. I recommended Sleight of Hand Pro
5. Perk 3 are random yet helpful. I recommend not putting Flak Jacket and Hacker together though.
6. Think of good combos. (Silencer + Ninja Pro.) (Lightweight + Marathon) (Hacker and Motion Sensor)
Step 8: Test It Out.
After building your class, go online and test it out.
Don't get frustrated about not doing good, it may take getting used to.