Select from menu, Options-Keyboard-Advanced-Enable developer console. From here on, every time you want to use the developer console press the ~ key(above tab) and copy paste some or all of the following commands. In game with the up-down arrow keys you can go through(and use again and again) the wanted command. Remember when you use a command, the first time enables it and the second disables it. Here are the correct line of commands that you mostly use.

sv_cheats 1;buddha;sv_cheats 0 (you can get hit to the last point of health but never die).

sv_cheats 1;ai_disable;sv_cheats 0 (paralyzes the ai of the zombies-sitting duck).

sv_cheats 1;ent_remove_all npc_zombine;sv_cheats 0 (remove all current zombies from map, more will be regenareted(until zero at forest,house,neighborhood).

sv_cheats 1;noclip;sv_cheats 0(fly,even through walls, repeat with the same command just a little above ground of place of map you want to continue on foot-dont get stuck in walls(but you dont die just use the command to safe ground and disable).

sv_cheats 1;givecurrentammo;sv_cheats 0 (gives ammo for weapon held).

mp_friendlyfire 1 (enable) mp_friendlyfire 0 (disable) These are for the heal achievement. Choose a map(with bots companions, from average difficulty and above. Start shooting a companion(or two) near your healing box(not to the death-red indicator) and let them heal till the box reach zero. Then restart map, do the same, use up all healing box, till you get the achievement. You can push your companions near the box. Trainstation map is the most safe of all to start.
Not all commands need the sv_cheats 1;command;sv_cheats 0 to work. They can be used directly.
But most do. All commands can be found on the internet. And rebember its the developer console. No cheat. Rate up and favorite this guide if it helped you.