You cannot install the original Counter-Strike 1.6 on Mac without Windows emulation software. You can however install Counter-Strike: Source, that's the revamped CS that has been ported to Source engine (the same one as used in Half-Life 2).
Requirements are:
Minimum: OS X version Leopard 10.5.8,
Snow Leopard 10.6.3, 1GB RAM, NVIDIA
GeForce 8 or higher, or ATI X1600 or
higher, Mouse, Keyboard, Internet
Connection
Update: As of 2015, I'd recommend newer Counter-Strike: Global Offensive. It supports Windows, OSX and Linux users on same servers.
You can't do this client sided, or at least, legally. A client side modification would be a hack and in the event of attempting to do this you will likely get VAC'd.
You need a server side plugin to do this for you. Whether you create a local game on your computer or connect to one online, you are still playing in a server environment. When you start a game on your computer it creates a server with your local IP and connects you to it.
The easiest way for you do do this for yourself playing with bots would be to install Sourcemod in your cstrike folder. It's in steam>steamapps>yourname>counter strike source>cstrike
There you will create a addons folder, inwhich you will put sourcemod, metamod, metamod.vdf, etc.
After download this plugin and place it in the plugins folder. [CSS] Weapon Mod
Set up the cvars/configuration files to your needs. It looks like you're looking for:
genericdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to the generic hitgroup. 0.0 stops the damage. This hitgroup is used by knife, grenade explosion and flashbang/smoke projectile physics damage.
headdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to the head hitgroup. 0.0 stops the damage.
chestdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to the chest hitgroup. 0.0 stops the damage.
stomachdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to the stomach hitgroup. 0.0 stops the damage.
armdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to both arm hitgroups. 0.0 stops the damage.
legdamage -- min value 0.0, max value 1000.0 -- Modifies damage done to both leg hitgroups. 0.0 stops the damage.
Then once you create a local server the plugin will load and you'll have your damage.
Best Answer
If friendly fire is on you can shoot through your teammate and damage the enemy, but the damage to the enemy is minimal. Your teammate will take full damage. If friendly fire is off your bullets hit your teammate and do nothing to the enemy.
It's generally a good idea to always move to a new line of fire and never stand right behind your teammate, but a bit to the side. It also helps if your teammate ducks during a firefight so you can shoot over him if you have to.