Here's some perk-specific advice.
I've already talked about going Sharp or Zerk on the first wave after you join. Going Zerk means using your knife; if it's several waves into the game you might want to avoid that. (Unless you're a high-level Zerk, but this is supposed to be a beginner's guide, yes?)
Medic: I have very little advice for Medics, as I've never played one. In general: try not to be a Battle Medic; your job is to keep your teammates healed. Fight when you have to, duck behind the other players when you can. Keep track of them, keep them healthy. You don't level your perk by killing -- you level your perk by healing.
The Medic gets a faster recharge on his syringe, more injection potency, better armour (starting level 1), faster movement (starting level 2), plus discounts on armour and a substantial discount on the MP7M "medic gun". Those all increase with his level. At level 5 he spawns with armour, at level 6 with an MP7M.
Support Specialist: Don't let the extra welding ability go to your head. But you need welding points to level. There's a way to do this.
- Join a solo game on an easy setting.
- Kill off all but a very few clots. One or two should be enough.
- Run through a door with them chasing you, but you need enough distance to start welding.
- Close the door and start welding it.
- While continuing to weld the door, put a brick on your "fire" button.
- The clots will continue to damage the door, you will continue to repair it. Check to make sure you're keeping it welded. If they're doing too much damage, unweld it, kill some clots, and try again at another door.
- Go to bed.
Next morning you'll have all the welding points you need to get to the next level. Maybe several next levels.
I actually bound the spacebar as a second "fire" key and used two old hard drives (one to balance, one as the weight) to hold the key down, but you can use whatever works for you.
After that you'll only need shotgun kill points to level, and you won't feel such a strong need to weld every damned door in sight.
All shotguns are 100% miss from point blank range. If there's a Scrake or Fleshpound in your face, either try to backpedal or crouch and aim for the head.
The shotgun is surprisingly effective, especially with the Support's penetration bonus. Reload obsessively whenever you don't need to fire continuously. Try to position yourself to hit several specimens at once, or use your sidearm for the small stuff if you can, to conserve ammo.
The hunting shotgun is a real sleeper. It's too slow to be useful for general shooting unless your other shotgun is out of ammo. But it's good against larger specimens, and the alt-fire, which fires both barrels, is particularly effective.
My favorite Scrake strategy for Support: backpedal while hitting him with slow fire from the other shotgun (careful to aim for the head!), keeping a distance of about 20 or 30 feet, until he rages. Instantly switch to the hunting shotgun, aim for the head (of course) and shoot with the alternate fire. If you did it right he should die instantly and you will be unscratched.
The AA12 is an awesome automatic shotgun with a drum of 20 shells. It does less damage per shot than the other two shotguns but has the best accuracy and the highest damage-per-second. If you do it right you can take out a Fleshpound without reloading. However, it has a long reload time and you can only carry up to 80 shells total.
The Support Specialist welds faster (starting level 1), gets more grenade damage (starting level 1) and more shotgun damage and penetration, can carry more shotgun and grenade ammo (starting level 1), can carry extra weight (starting level 1), and gets discounts on shotgun weapons. At level 5 he spawns with a pump-action shotgun and at level 6 he spawns with a hunting shotgun.
The Commando's weapons are the Bullpup, the AK-47, and the SCARMK17. He can see Stalkers and the Patriarch when they're invisible, and he can see specimens' health bars. To level he needs to get kills with his perk's weapons and Stalker kills.
Commando is a mixed bag. I'm told that at higher difficulty settings he's not even considered worth playing. However, he's good at rapid fire, he can spot things others can't easily see, and the SCAR can be very useful in his hands. He's a good mid-weight perk and probably the easiest perk to play.
While his weapons can fire rapidly, in general I highly recommend ammo conservation. If you're holding the firing key down and there's not either a horde or a tough specimen in front of you, you're throwing away ammo. Short bursts aimed at the head are best.
Bullpup is one of my favorite weapons! It's lightweight and not very powerful, but it has a low recoil and is very accurate. I like it very much for use against small specimens. Use the iron sights, aim for the neck, and stroke the trigger. It will pull up and headshot the specimen. Aim at the next specimen and repeat. If you set it to semi-auto mode it will handle the short bursts for you but for myself, I find it easy to control the trigger well enough that I don't bother. Once you get the technique down you can quickly snipe hordes of specimens very efficiently at medium range. If too many get up close, switch to automatic and just hold it down against the recoil. I find the iron sights to work well in most conditions. In spite of its lesser damage, in later waves I may keep it in favor of the AK-47 simply because it's so easy to use. It's also a great backup weapon for other classes.
The AK-47 is the Russian weapon we've all come to know and love. As compared to the Bullpup it has a slower firing rate, less ammo, more recoil, and in my opinion a crappier sight; however, it does a lot more damage. If I feel my team is a bit weak I may keep it in favor of the Bullpup in later waves due to its better fire-power.
The SCARMK17 is a thing of wonder. It has a very nice scope, does more damage than either the Bullpup or AK, has a higher rate of fire, weighs less and is more accurate. But it also costs more, has more recoil and a smaller magazine.
The best thing about the SCAR is that you can use alt-fire to set it to semi-automatic, which gives you a single round per trigger pull, sight in with the scope and rapidly single- or two-shot most small specimens. As with the Bullpup you can clean up large amounts of specimens, but at slightly longer range as well. It basically turns your Commando into a half-decent sniper for the little stuff.
The SCAR is light enough to make it one of the few weapons the Sharp can carry along side of the rather heavy crossbow. For myself, I've tried it and I prefer the Handcannon.
A high-level Commando can take out a Scrake solo but it's very expensive, and will cost at least one magazine if not more. And then he'll have to reload. Against a Fleshpound he's better off supporting another, more appropriate class, or ganging up on it with other players. If there's a decent Sharp or Commando or Zerk available I'll report Scrakes and Fleshpounds as I'm running away, and only engage it directly if I have no choice. However, I can be very effective at supporting him by wiping out the little stuff while he takes on the big guy.
The Commando gets extra damage and discounts on his perk's weapons, increased magazine size (starting level 1) and increased reload speed. These increase with level. He can see cloaked specimens and (starting level 1) can see all specimens' healths. At level 5 he spawns with a Bullpup and at level 6 he spawns with an AK-47.