Once in arma you will need to reconfigure some keys so that you dont use the in game chat. While in a game in arma, hit escape then Options then Controls. Delete The Keys assigned to “Voice Over Net” and “Push to Talk” [in both cases they will be Caps Lock and Caps Lock x 2] delete both of these as below.
By default in arma you start off with a AN/PRC-343 Short range radio (between 1 and 2 clicks coverage), you can see this by pressing CTRL+SHIFT+V this is a local radio more suited to Squad Coverage and is poor + Non effective at range. To Transmit Press Caps Lock and talk, an indicator down below like the following will be displayed:
Long range Radio (SINCGARS [Single Channel Ground to Air Radio System])
In order to command or communicate over a longer distance you will need to pick up a Sincgar set from a gear box at the spawn point (Normally a big silver box), with the sincgars equiped you will be unable to equip any rpg or rocket launchers. The designation in ARMA 2 with ACRE for the Sincgars is: AN/PRC-77 go ahead and pick this up and equip.
Using CTRL+Shift+C you can switch quickly between both radios, using CTRL+Shift+V will allow you to select manually which radio you will actively transmit on, you can also increase signal power on the set to compensate for urban or longer distances to improve signal quality.
Transmitting with this set is the same as the other, it may be an option to press CTRL+Shift+[Direction Arrow] on the PRC-343 to keep it on one ear and the PRC-77 on the other.
People with PRC-77 sets will be able to hear and transmit to people who are near enough with the PRC-343 sets, remember though people with PRC-343 Sets will not be able to hear PRC-77 sets that are out of range.
A squad should always have 1 person with a Long range set, so that that person can speak to other teams that may be in the air or back at base.