Jack of all trades
Honestly, I try not to specialize armies against specific types of opponents. It takes some fun out of the game.
The generic answer is you should design an army that fits your style of play.
A more helpful answer is that in 40k, especially with armies like orks, you should start with what's required (1 HQ, 2 troops) and go from there. With any army, I usually spend about 1/3 to 1/2 the points on troops, and with the orks, it's usually always 1/2 or more.
With ork troops there are two popular choices: 12 boy trukk squads or 30 boy mobs. In either case, if you intend the unit for close combat, you need a nob with a power klaw, and a bosspole doesn't hurt either. Although that's 40 points more per mob, that unit will now be able to kill anything, even monstrous creatures and armor. Just two days ago I killed two Chaos Princes (monstrous creatures) and their entourages in close combat with two mobs of ork boyz. Without the klaws, I probably wouldn't have even dropped them below half.
Don't have nobs and powerclaws? Just put them together out of spare parts. Honestly, the more odd and ridiculous it looks, the better (at least for orks ^_^).
I find it's best to mix the two strategies. My 1500 point armies generally have 3 trukk squads and 2 footsloggers (the general term for close combat squads that walk all the way to the enemy, in case you didn't know). In 2000 point armies, I add a sixth troop of either type.
Why are ork troops great? Because it's the only part of the army that's really difficult to kill. 30 fearless boys (until they drop below 12, and even then they're virtually fearless until they hit 8) is very hard to stop, even with ordnance, and even if only 12 slugga boyz make it to close combat, that's still 48 attacks at S4, WS4 and I3. That'll kill about 3 marines or so, and a power claw on top of that will kill 2 more. Assuming you killed 1 before close combat began with pistols, you just erased a team of 6 marines off the board, and if it was a squad of 10, they'll need to make a leadership test, most likely at -2.
Trukk boyz can work even better, as your trukk can move 12in, you disembark 2in, and then you can assault 6in. If you WAAAAGGHHH, then that's another d6 movement instead of shooting. That's 20in movement. Granted, it won't be as fearsome as 30 boyz, and a shooty army can easily take down an armor 10 transport, but if you hide them effectively, this is no problem, and even if they blow up, trukks have special rules to protect the passengers, when they explode.
Shoota boyz are also great, especially for mobz. They have one less attack in CC than sluggas, but their guns have 18in range, and are Assault 2! For 6 points, that's a steal. Granted, there are some uncertainties with the new rules, so it's unclear if a nob with a shoota squad can take a power klaw. Right now, I run them nobless (but never in trukks).
The troops are fearsome enough in close combat for anyone, so I find the rest of the army should either be good at shooting, to support them, or be able to ensure that your ork boyz are the ones to assault the enemy, rather than the other way around. If your opponents are the first to assault you, then your troops are going to have one less attack, strength and initiative each, and depending on what army you're facing, this can mean the difference between winning and losing.
Lootas, warbuggies and bikes are great for anti-infantry, and aren't bad at anti-armor either, as long as they can hit their side or back. Don't spend more points than you can lose, however, as your opponent will almost certainly destroy them first. As long as your troops make it to CC, this shouldn't matter. For this reason, warbuggies should be taken alone, instead of in groups of two or three, to minimize losses when destroyed.
Tankbustas are great for taking down armor, but generally need a transport to be effective. I usually run small units of them in looted wagons, but other people have success with large units in battlewagons, so it comes down to taste. Again, don't be surprised when your opponent throws everything they have at them.
Stormboyz, Kommandos, and all-nob units are great in close combat, but no matter how tough you make them, your opponent will be able to wipe them out in one turn if they put their mind to it, so use them carefully.
Battlewagons are powerful, with front armor of 14, and great weapons, but they're expensive. I prefer expendable looted wagons for less than 1/2 the cost, but it comes down to personal taste.
Ork HQs are very versatile, so it comes down to what you need:
Mobile fire support/tankhunting: Warboss on bike, or the special character equivalent
Powerful ordnance: Big Mek with Shok attack gun
Close combat power: Ghaskull (spelling?), or Mad doc Grotznik
I don't have any experience with weirdboyz, so it's up to you whether to use them or not. The same goes for the artillery and flash gitz. All in all, I think they're expensive for what they do, but that's just my opinion.
When playing orks, no matter how you build your army, your troops will be what win or lose it for you. The other units are nice, and very destructive, but too fragile to be depended completely upon. I know people play different in tournaments, but tournament play is very different from casual play. People in tournaments tend to have an all or nothing approach that may be a 'best bet' in tournaments, where who goes first tends to determine who wins, but those armies generally aren't fun for casual play.
So start with the troops you like, and then envision playing with them. What are they going to need in order to survive long enough to get into close combat? Are they going to need fire support? Are they going to need a powerful boss to fight with them in close combat? Do they need an insane medic to ensure they survive whatever is thrown at them? Are they going to need something to blow up the enemy tanks?
Above all, do what's fun for you, but have faith in the slugga boyz and power klaw nobz. There is virtually nothing they can't kill.
