1. Before ever playing, go into the control settings menu and LEARN THE CONTROLS. This is the biggest question I see asked, people wanting to know what button performs certain actions. The control settings menu has the answer to EVERY one of these questions.
While you are at it, go to kf-wiki.com, and learn the name of all the ZEDs, as well as their abilities. This is more information that is invaluable to playing properly.
2. Do not buy weapons your class does not support...to an extent. If you are a Medic, Demo, or Berserker, it's okay to buy a Handcannon/LAR/Crossbow. These weapons somewhat help alleviate the weaknesses of these classes. However, if you are a Sharpshooter, don't be buying a Shotgun, and if you are a Support, don't be buying an Assault Rifle. For the most part, this is common sense, but for newer players, it will come with time.
3. Do NOT attack a Scrake or Fleshpound without first determining the proper course of action to take. If you see a Berserker going after a Scrake, NO ONE should be attacking the Scrake. The Berserker is literally the anti-Scrake, mainly because most of his attacks can stun Scrakes. He can solo Scrake's, taking little or no damage in the process. If you are attacking a Scrake that the Berserker is attacking, you are un-stunning the Scrake causing the Berserker to take a lot of damage, and you are also wasting ammo.
Fleshpounds need to be handled slightly different. For one, the Berserker can't stun Fleshpounds, so if you see him trying to solo one, help him out. Number two, Fleshpounds are very weak to explosives, so use them nades, but don't get yourself killed. Third, don't try to be a hero, because a Fleshpound will quickly show you how not gangsta you think you are.
4. If you need to close distance on a Husk or the Patriarch, don't walk in a straight line towards either one. If you strafe left and right while walking, you will avoid virtually anything these two ZEDs shoot at you. If you don't strafe while walking towards one of these ZEDs, be prepared to lose all your armor, and almost all your health. In the case of the Patriarch, just be prepared to die.
5. For the new players who want to play Firebug, there are a couple of things you need to know. The first thing is, your main job is to set everything on fire(except Scrakes and Fleshpounds), NOT TO KILL EVERYTHING! Like I said in parenthesis, do not set Scrakes or Fleshpounds on fire, as this just fills their rage meters, and it WILL cause them to flip a wig. If you are a level 3 or higher Firebug, you need to STOP using your grenades on Fleshpounds, and to a lesser extent, Scrakes. At level 3 and higher, your grenades set things on fire.
Against Husks, as a Firebug, you need to intercept their shots. Yes, that means you need to deliberately get hit by them. At lower levels of the Firebug perk, it is probably a better option to dodge them, but only do this if you are alone, and no one else is around. Also, attack Husks with your MAC-10, because they are highly resistant to Fire.
Finally, with the Flamethrower, never hold down the primary fire button. All this does is waste fuel. One tap is all that is needed for all ZEDs you wish to set ablaze. The only time you should hold down is if you get pinned somehow, and you need them to die quickly, or if you are trying to burn down the Patriarch.
These five tips above are the best advice I can offer new players. Now, here are some other tips I want to offer, and they are something that EVERYONE should follow, new and old players alike.
-Never use the term Noob, Newb, Newbie, Nub, etc. It's condescending anytime it is used.
-At the end of every round, if you have everything you need, donate the money you didn't use. This is a team game, and money is not a precious resource. Give priority to players who just joined, or have the least amount of kills.
-Being good at ANY other FPS does NOT make you good at Killing Floor. KF is a whole other monster.
-Be sure your perks are of sufficient level for the difficulty you want to play.
-Flamethrowers and grenade smoke impair vision greatly. If you see a Scrake or Fleshpound, avoid using such weapons if a Sharpshooter is present. The Sharpshooter can kill it a lot quicker than you can, but only if they can see the head.
-Finally, this is a team game, and it should treated as such. Just because you are John Rambo during solo play, does NOT mean it's an effective strategy with other people playing. ALL of the ZEDs get stronger, and gain more the health, with the more people that are present. You need to know everyone's role, and coordinate accordingly.