Okay, so we've established that you're a newer gamer and don't have $500 in disposable income to rearrange your collection.
That said, working with what you've got, and what you might reasonable acquire soon...
HQ:
Warboss with powerfist
I'd give him some sort of better save. A Cybork body, at least, will help out with some of the damage he takes. You also have to decide how he gets to the fight. If he's running with 30 boyz (and only 30), there's a good chance that they'll get shot up before they get there. I'd let him ride in a vehicle, if possible.
Also, you say you have a mek with a forcefield, and that's vital to most ork forces, so run him.
Troops:
30 a'rd boys with sluggas and choppas (3 with big shootas) and a nob with a powerclaw and waaagh! banner
You could probably run these guys as normal boyz without losing much, as long as you keep them in cover as they advance. Hiding behind the advancing battlewagon could help here.
11 ork boys with shootas (1 with rokkit launcha) and a nob with powerclaw
This mob is the right size to fit in a trukk. Not a bad place for it. Although, with shootas instead of sluggas, you will probably have a hard time in assaults, especially if they go past the first round. Maybe use the trukk to drive them to some cover and camp out there shooting stuff? Also, give the nob a bosspole, it will stop them running if they get shot too much.
10 grots with a rutherd
10 grots with a rutherd
10 grots with a rutherd
Divide them into a unit of 18 grots and 1 slaver, and a unit of 11 grots and 1 slaver. The unit of 11+slaver's job is to hide if possible, or distract the opponent. The other unit will sit with the mek.
Heavy support:
Battlewagon with ard' case, killakannon, grabbin klaw, 2 rokkit launchas, armour plates, reinforced ram, and 4 big shootas
This looks like a shooty wagon. That means you can't assault out of it and probably don't need to get up close to your opponent with it. It wouldn't be a bad bunker for the grots if you cut their numbers down (18 grots, 1 herder) - and it would mean that the battlewagon could control an objective. If the big mek joins this unit, he can project his forcefield from the hull of this tank, and protect it as well.
I'd lose the grabbing klaw - there's no reason for this wagon to get that close to your enemy.
Deff Dread with 4 dreadnaght close combat weapons and armour plates
Quite serviceable.
Fast attack:
3 ork warbikes with nob with big choppa
If you still have the parts, cut the big choppa off the nob and give him a powerklaw. There is no reason, ever, to have a nob leading a unit without a powerklaw. Also, give him a bosspole, as with a powerklaw, he can do some decent damage by himself, if he's not running away.
Elites:
5 nobs with big choppas
5 burna boys
And, here are the odd men out...
5 burna boyz or 5 nobs are just targets. They're fairly dangerous, so your opponent will want them dead, but they're hard to protect. These are the weak links in the army as you have it now.
I'd recommend buying a box of nobs and another trukk. Build the other nobs so that you get at least three powerklaws, give one a waaagh banner, one a bosspole, and one a kombi-skorcha. And, if possible, do some sort of conversion to make one into a dok. Give them cybork bodies, and another trukk, and let the warboss ride into battle with them, and you'll have a unit that can really hit hard. And, because you've got the warboss, you can make them troops instead of elites, and they can also score objectives.
As for the burna boyz, they're a hard unit to make work right. Ideally, you have a good number of them and ride them in an open-topped battlewagon, so you can ride up and hit enemies with a lot of templates, or charge with a lot of power weapon attacks. You might consider leave them on the sidelines until you can buff them up like that.